This has been a really interesting thread.
I am an overseas purchaser, and have bought fabric from the States for over 10 years now. Over this time, and with my interactions with other local quilters I have learned that there are 2 types of overseas purchasers; those that want to maximize the postage (e.g. cram in as much as you can), and those that want something so specific they will pay a fortune for postage.
You can never know what type of customer you have.
From my personal experience - I would value something on my customer profile (don't know if you store has them) where I can state what's important to me. For me, I want to cram as much into an envelope. If an item is not available I would want the option to look for something else to fill up the envelope before shipment.
What I must also say is that when I am shopping online, I ALWAYS check the shipping page and the postage rates, how many yards (of standard width 40-44" cotton fabric)will fit, and the other terms and conditions.
I am also fully aware that my country may charge additional levies/taxes, etc.
If I have any questions about my order, e.g. can I buy 7 yards, and can you squeeze in a ruler, thimble, pencil (whatever) would it be included in the postage amount? I ask BEFORE I make the order.