What's appropriate to say?

Your message is just as polite as can be; it's hard to see how the average Jill would find it offensive. I think your first buyer ("She said she didn't want to do business with such a nasty seller and refused to pay.") was looking to back out anyway. The second ("She paid- but also mentioned she didn't appreciate my message.") is a little strange. Perhaps she didn't read the auction terms, and assumed she had 7 days to pay? Judging from the requests I get after the sale, I've concluded many people don't read before they bid. Or maybe, as one writer said, you just won the lottery twice—you lucky person!
 
Your message is just as polite as can be; it's hard to see how the average Jill would find it offensive. I think your first buyer ("She said she didn't want to do business with such a nasty seller and refused to pay.") was looking to back out anyway. The second ("She paid- but also mentioned she didn't appreciate my message.") is a little strange. Perhaps she didn't read the auction terms, and assumed she had 7 days to pay? Judging from the requests I get after the sale, I've concluded many people don't read before they bid. Or maybe, as one writer said, you just won the lottery twice—you lucky person!

I totally agree, you just got unlucky 2 times in a row.
Hope things pick up for you!:yes:
 
Your message is really nice and polite, I cannot see any normal-minded person being offened by it. Some people just don't want to admit they are wrong and cannot read a seller's checkout instruction.

I just sent a non-paying bidder a reminder, now I have a feeling she is going to e-mail me saying she hates doing business with me also.