Interest depends on whether or not you pay the full amount on your credit card every month.
For example:
You purchase an item and the cost of the item, plus tax, plus shipping equals $30 and the easy pay for that day is 3 easy pays.
QVC bills your card $10 per month for 3 months. If you pay the $10 then there is no interest charged for that month by your credit card company. If you don't pay the $10 then your credit card company will charge you interest and it rolls over into the next month.
It's just like any other credit card transaction. If you don't pay the full amount on your monthly credit card then you are charged interest by your credit card company. If you pay the full amount every month then there is no interest because you have paid the monthly bill in full. QVC doesn't charge you interest, the credit card company does. Hope this all makes sense and helps.