Flat zip pouches are useful because they sit neatly inside a tote while keeping small essentials easy to find. I look for a wipeable exterior, a slim profile and a zipper that glides. For more on the broader bag conversation, see Louis Vuitton, plus these Phia edits on a small pouch or cosmetic case.

First, the coated pouches. I would start with the Marc Jacobs Beauty Bag in red, a pebbled coated-leather case with a removable wrist strap and a tactile finish. The Katie Loxton small black case has a smooth pebble finish and a gold-tone zipper, and it opens flat. TRVL Design's Woven Jet stands on its own and has a wipeable finish. For something glossier, the burgundy Vault from Urban Essentials has a structured base and a lock-accented pull. In October I would pick one in black or burgundy and keep it in a work bag. The test: run your thumb along the zipper. It should glide, and the metal should look warm, not brassy-bright.

Leather gives a zip pouch a polished, tactile finish, whether it is soft nappa or grained. This group is for anyone who wants a slim outline with a little more structure than coated fabric.

The ROYCE New York Leather Zip Travel Pouch in cream is the flattest of the lot: nappa leather, a long top zipper and a water-resistant lining, only a quarter inch deep. It will not hold a full routine, but it slides into a handbag without bulk. The Shinola small zip case in dark brown comes in pebble-grained leather with a wristlet pull and four interior slip pockets. For a color, I would look at the emerald green Flaire pouch from MAISON de SABRÉ, or the green full-grain case from Casupo with a gold zipper and key ring clip. HOBO's large black crinkled pouch is top-grain Italian leather with a hidden magnetic snap, so it reads sleeker than most.

Pair these with a camel coat and a gray knit, the October uniform. The buying test is the lining: tap the inside. Water-resistant or twill cotton lining is easier to live with than bare suede, and a quality bag should have it. Look at the zipper tape too, and check the pull is leather or smooth metal, not a plastic tab.

What I would skip: the larger toiletry bag in this group if you want a flat shape. It is a good bag, just a different silhouette.

A slim zip pouch earns its place through proportions: a long zipper, shallow depth and clean corners. Anything puffy takes up more room in a tote.

The Kate Spade New York Mini Zip Card Case in black is a slim pebbled-leather zip case with a gold-tone zipper, small enough for a few cards. MZ Wallace's Metro Accordion quilted pouch in black has a top handle and a two-way top zip, with a diamond quilt that stays tidy. Loeffler Randall's Georgette in brown gingham has a flat base and a label patch, while the Frances is a quilted brown-and-black check. The Béis small pouch is lined polyester with a polyurethane coating, plus elastic pockets and two loops, so it is the practical pick of the group. For a lighter look, the natural woven Mae from Stephanie Johnson has faux-leather trim.

A washable lining is a useful detail to check. A makeup pouch spends its life next to powder and liquid, so Glamour's makeup artist advice is to look for a wipeable interior, strong zippers and a firm exterior. That is a practical checklist for any of these.

The fitting-room test, or really the tote test: put the pouch in your bag with a phone and keys. If it holds its outline, it is built well.

What I would skip: the baggallini small pocket case if you want a perfectly flat outline, since its front zip pocket adds a little depth. It is a useful bag, just a slightly different shape.

Pouch sets make sense if you want the same compact shape in three sizes: one for makeup, one for chargers, one for the passport.

CALPAK's three-piece set in black is water-resistant polyester with side carry handles and graduated sizes, which makes it the neatest answer for a carry-on. The KUSSHI On the Go set is two leopard-print neoprene zip-around pouches. POP UPS BRAND's navy neoprene pair nests, has mesh dividers and is machine washable. If you like a more tailored finish, MZ Wallace's Metro Packable set is quilted black with leather trim. For softer colors, I would look at the Go Pouch sets in Jewel Mix or Candy Shop Stripes, both in a technical weave, or the green gingham trio from The Somewhere Co with a water-resistant coating.

Byrdie quotes makeup artist Jamie Dorman: “Ideally, the bag will be resilient, waterproof, and easy to wipe down.” I would apply that to sets in particular, since one piece usually carries the messier products. Put the liquid things in the coated pouch and the dry ones in the canvas one.

A good test is the nesting: slide the small pouches inside the big one. They should fit with room to spare, and the zippers should not snag on each other.

What I would skip: the unlined Go Pouch sets for foundation or serums, since a spill is harder to wipe out. They are better for cables and cards.

Travel cases for beauty essentials work best with a little more structure, whether they are headed into a holiday suitcase or onto a hotel counter.

CALPAK's small clear case has transparent panels, a two-way zip-around and a top handle, which helps at the airport. The Dagne Dover Hunter Large in pink comes with two removable zip pouches, elastic loops and a mesh pocket, and it is made of Repreve recycled polyester. Rains makes a minimalist waterproof one in black. KUSSHI's Large Neoprene Train Case holds 30 to 35 products, and the Béis Cosmetics Case has a textured faux-leather exterior that wipes clean. For a more dressed look, ETOILE's beige vanity case is saffiano-style with a removable organizer, and TUMI's Madeline in rosewood has leather-wrapped top handles, a detachable interior pouch and a metal hanger. MZ Wallace's navy accordion case is quilted with leather trim.

Before a trip, I would pack the case with your real routine and zip it closed. If you have to push, go up a size. A makeup artist quoted by Glamour recommends a firm exterior or reinforced sides, a quick guide to whether a travel case will protect a compact.

What I would skip: the hanging case if your bathroom has a lot of counter space. It earns its place in a small hotel bathroom.

I would begin with the shape. If you love a flat coated pouch, choose one from the first or third group in a dark color and look at the zipper and lining before anything else. If you travel often, add a set so the small things stop floating in your bag.

Then I would think about the trim. Leather edges and a warm gold zipper bring polish to a clean silhouette.

A pouch that is simply well made, in your own color, quietly does the job. If you want to keep browsing, I also put together Card Holder.