Best Buy - HSBC Retail Services Purchased washer and dryer on one year same as cash, they now say I owe interest and late fees Wilmington Delaware

I purchased a washer, dryer and refrigerator at Best Buy. I was going to pay cash but the sales associate said I could charge it and if I made the minimum payments and paid it off within a year, I would not have to pay any interest. I did this and later received a bill for the interest. I spent hours on the phone explaining the situation and sent them a copy of the receipt that explained this. They still say I was only to receive a reduced interest rate even though the receipt clearly states that it is a no interest charge. They have since sent this to collections and basically ruined my credit.

Sher
Mount Vernon, Washington
U.S.A.