Going back to my mention of woodblocks,(another thread) an early mistake I made was sizes. One picture I created I could print to my paper size, but the customers ended up having to buy custom made frames, adding about £80 to the bill! So I learned that if I was expecting them to buy the frame and mat, then I had to make my images to easily bought off the shelf frames.
What you say about selling in mats is good. If you buy off the shelf mats (we have bargain shops that do this...and craft shops in the uk) it will cost an extra £3 but you can add an extra £10 to the final product because of the worth it adds.
However that also adds to the postage costs.
It’s worth thinking in terms of pricing :
What will make it sell better
Pricing at a value that makes it a good deal
Pricing at a value that puts it behind the reach of an after thought
Sell people the process and not just the image
My prints sell without a mat for £30 each. £5 for a Mat
£5 more for postage
They cost me £2 to make each one. And my time is covered by all sales easily.