As an online shop owner, I'd give the following advice to simplify your life. (take it or leave it)
8 yards in a flat rate envelope is about double what can comfortably fit in those without; a) violating USPS rules, b) costing you a ton of time trying to wrassle that much fabric into them, and c) risking damage to the fabric. A couple of suggestions:
1. There really is no competitive advantage to over-stuffing FRE's, especially when your orders come back as damaged. Cut your maximum yardage for FRE's down, your orders will pack up quicker and you can protect them better, eliminating damage returns and/or complaints.
2. Use 1st class International parcel for your smaller orders, up to 4 pounds (about 12 yards+). It's just as cost effective as FRE's but a lot easier to deal with, the customs papers are simpler and faster, and it takes the same transport as Priority mail does, so it gets there in about the same time. Of course, this may require that you change your cart shipping calculations which takes some setup work. You'll also have to buy your own containers.
Don't sell what you don't have in stock. That eliminates your problem with pre-orders and back-orders. With manufacturers canceling lines right and left nowadays, pre-orders are becoming more problematic, and backorders are always a problem, as you're discovering. If you oversell a fabric, i.e. you have 2 yards in stock and the customer orders three, contact them and ask them if they want the 2 or if they want to cancel. I use skype for international calls to customers, it's cheap and works well, but you do have to check and see what time it is where you're calling before you call.
FWIW... :mrgreen: