I think the flat rate postage of £1.50 per order is a great idea, and not at all expensive when you can order as much as you like!
One thing I did notice - the reindeer ribbon is listed as 4 metres for £2.50, which is a good price, but the title on the listing doesn’t mention the 4 metres, so I would assume the price is per metre if I was just looking quickly. It might be worth adding the actual amount in the titles as well as highlighting it in the descriptions…or have a consistent length for all ribbons, and make it very clear.
Searching for supplies on here is also a bit of a challenge. I recall searching for something ages ago and giving up because there was no way to filter the results and I was trawling through hundreds of listings! It may have improved since, but I would think that tags and keywords are really important for supplies.