This is your business. Make some contracts on your computer that state YOUR terms (I think there are some quilting and crafting books on this. Check you library)
Be as detailed as you can.
Yes, I think you're right. She never intended to do anything but take advantage of you. You appeased your customer by dropping your price. Did you have to do that with the mailman? No, he appreciated your work and paid for it.
Don't do business with her again. Even if she bad mouths you so some people, they probabaly already know her.
Stick to your detailed contract, which will be signed by you buyer before any work is done.
You can even say in the contract that if the item is not paid in full and picked up within 30 days, the quilt is on the market and for sale, NOT ON sale.
I like your idea of paid in full at the beginning you can have acontract written up for that alos. You may find it in your best interest to keep photo records of each quilt you sell, so there can be no quibbling about the condiiton of the quilt later. You're not Sam's Club with returns. You are a craftman. When you value yourself for your skills, others will too.