So last cycle I paid my Spark Business card off just in time to avoid interest.
Fast forward to today, I check my account and saw there was a minimum interest charge of $0.50 so i call up and get a very nice rep in Portland, who i asked why I was charged interest, she took a look at my account and saw i did pay off my last balance completely so she went ahead and credited my account the $0.50 but she did note that with Business Cards they need to be paid off in full for 2 full months to avoid interest, but she was able to make an exception for me. She also put in the system not to charge me interest my next cycle, so i will be free and clear.
Just thought I would let you all know, that if you have a Promo APR, 2 months before it ends, start paying off your account in full as if it ended 2 months early and you will avoid seeing this interest that i had.
Just a warning for you guys! 
Thanks for sharing...