I just got off the phone with BofA.... I am not thrilled... I had a promotional APR on my card for purchases which expired on 3/4/16. A few months ago they sent me a BT promo offer. I took advantage of it. I made extra payments to make sure that my purchase promo balances was paid off prior to the expiration of 3/4/16. I PIF this balance in 12/2015 or so I thought. I noticed that I had been charged interest this last statement period. After a failed chat and two reps over the phone I ultimately learned that I was set up to pay interest. Apparently, when I make payments over the minimum payment, they are automatically allocated to the larger balance. Obviously my BT of $2000 was larger than my balance of $800. So when I made my payments to cover the purchase balance and then some these payments were applied to my BT. The rep said these payment allocation rules are set by the gov't and even if I called I would not be able to indicate which balances I wanted my payments applied to. I find this difficult to believe because I have never had a problem doing so. However, my experience is limited to my Amazon Store Card and other store cards. Anyway, now that I am being charged interest I get to pay off this balance first. Needless to say, I will not be taking any of their BT offers, making any purchases until the current BT offer is up... I don't want to be told of a new rule later...
NFCU cash $25k | BofA Custom Cash $25k | Amex BCP $18k| Costco Anywhere $14.5k | Apple Card $5.25k | Cap One QS $4.5k | Sephora Visa $4k | Discover It $3.5k | Citi DC $3.5k | Citi AA Mile Up $3.1k | Chase Amazon $3k | Amex ED $3k | Chase IHG $3k | Chase Freedom Unlimited $2.5k | Chase Freedom $2.5k | Citi Rewards $2k | Last HP 8/16/22
Thanks baller... I don't either but apparently I shouldn't put a BT on a card w/ a balance either. Whether that balance has a promo on it or not... Lesson learned...
The only "rules" I know of set by the gov. is that any payment over the minimum due is supposed to go to the balance with the highest interest. I've never heard of a rule that it has to go towards the highest balance. Sometimes I think they make up rules as they go 
Anyway, I usually do what baller4life suggested, the only time I don't is if the purchase APR is 0% too and it expires at the same time as the BT APR.