Synchrony Financial Credit Card Complaint

Synchrony Financial Credit card department,

Payoff process Michigan

Made purchases based on no interest promotion. Paid regualrly. Made additional purchase based on no interest promotion. Believed long period of no interest based on promotion expiration of 2018. Paid regularly. One promotional period expired and the payments we believed to go toward first purchase pay-off, went to both purchases so balance left on first promo and ridiculous back amount and continual amount of interest has accrued - $500.00 on initial $1500.00 purchase. In the midst of pay-ff to GE Capital, account changed to Synchrony Bank w/o our consent. This helped to mask the confusion! Is it a common trick? Please help!