So am I! It has worked for me before, why not now?

I've also had a $100 bill that I split into 4 $25 charges. Surprisingly 2 of the charges posted, but the other 2 did not! I had to FAX in the 4 receipts to get the other 2 credits. (This was before I knew about the "varying charges".)

Perhaps, the computer goes by the amount of miles given. So if you get 5/$, then $15.00 and $15.10 would both give 75 miles, but if your tip causes the charge to be $15.20, you would get 76 miles. Maybe the computer would not think it is a duplicate.