Is paying full price considered an interest payment

I bought a jar of ointment from a lady who only used it a couple of times. Do I have to pay her the full price for it, or maybe on the contrary, this is forbidden because it’s considered paying an interest? And if this is problematic because of interest, how am I supposed to pay her?  

Dear …!

Paying high price for a product does not present a problem of interest because this is a sale, not a loan. But according to the laws of buying and selling, you don’t have to pay the full price in this case, since a used product does not cost as much a new product. It should cost about 30% less.