Every autumn a customer comes in holding a phone, showing me a photo of a bag she has seen online, and asks whether it will go with the coat she bought from us last year. That is the real question behind every trend list, including the Cosmopolitan's rundown of the five bag shapes they reckon you will see everywhere this season. The shapes come and go. The coat, the boots and the weather stay the same.

So I am not going to argue with the magazines. Slouchy shoulder bags, big soft totes, suede, top handles, something small and structured for evening: that is more or less the spread every autumn, and it is a sensible spread. What I will do is tell you what happens to those bags when you carry them from a car park in Ballymena to a restaurant in Belfast in September, because that is the part nobody photographs.

Rain is the trend nobody mentions

Suede is the one that catches people out. It looks magnificent in a lookbook and it looks magnificent for about three weeks here. Untreated suede and an Antrim downpour do not get along. If you fall for a suede bag, and plenty of people will because it is genuinely the nicest texture going this year, buy a protector spray at the same time and use it before the bag ever leaves the house. Two light coats, let it dry properly between them, repeat in December. That is a five minute job that adds years.

Smooth leather is the safer money. A grained or pebbled finish hides scuffs far better than a polished one, which is why I steer customers with young children or a long commute towards grain every time. And check the base. A bag with feet on the bottom, or even a stiffened panel, will survive being set down on a wet pavement. A soft unstructured base will wick water straight up into the lining.

The other thing to look at is the hardware. Cheap plated hardware on a bag that lives in Northern Ireland fashion weather will pit and go cloudy inside a season. Feel the weight of the zip pull. If it feels like nothing, it is nothing. A heavy zip and a stitched-on strap anchor tell you more about how long a bag lasts than the label does.

The shapes that actually earn their keep

Of the shapes doing the rounds, the big soft tote is the one I would put money on for most women here, because it does two jobs. It carries the day, and if it is a decent leather it dresses up enough for dinner. Look for one with a proper top closure rather than an open mouth. Open totes and side-on rain are a bad combination, and you will be forever fishing damp receipts out of the bottom.

The slouchy shoulder bag is the fashion pick and I like it, but it wants a considered outfit. It sits high under the arm and it fights with a bulky puffer. Worn over a wool coat or a good knit it looks effortless. Worn over a hooded technical jacket it just looks like you are carrying shopping. That is worth knowing before you buy, not after.

Small and structured is the one people think they cannot justify. I disagree. Half the Belfast style problem at a wedding or a Christmas do is a woman in a beautiful dress carrying a bag built for a laptop. A compact top-handle bag in black or a deep berry will get used ten or twelve times a year for the next decade. That is not an indulgence, that is arithmetic.

What I tell people over the counter

One, buy for your coat, not for your dress. Your coat is what the bag sits against most of the year. Tan and camel leather against navy and grey wool is the easiest win in womenswear Belfast has going. Black on black is fine but flat, so lift it with the hardware or a bit of texture.

Two, strap length matters more than trend. Try it on standing up, with a coat on, over the shoulder and crossbody if it converts. If it cuts across your chest at an awkward height it will annoy you into leaving it at home.

Three, spend where it shows. A mid-price bag in honest leather with good stitching beats a designer-shaped bag in coated fabric every time. And for the men reading, the same rules apply to a holdall or a work bag. A waxed cotton or grained leather holdall is the single most useful thing in menswear Belfast that nobody thinks to buy until they need it.

Running an independent clothing shop Belfast customers travel to means I see the same bags come back years later, still going. That is the test.

If you want a second opinion on whether something works with what is already in your wardrobe, call into us at 2 Killane Road, Ahoghill BT42 1RA and bring the coat with you. Or have a look through the shop online and we can talk it through from there.