Amazon Returns (Third Party Seller)

[AeroK]

I'm trying to find advice about returning a damaged/not as described item on Amazon to a third party seller. The item was described as new and sealed but when I received it, it was missing pieces and obviously not sealed. The seller has approved the return and I'm about to print the shipping label but I see that I will have to pay for postage. The item is heavy so postage will be high. I'm starting to question whether or not it's worth the hassle to pay to ship it back to the seller. How are refunds issued? Does it start processing once the seller receives the return? Should I instead file a claim? The seller seems to be hard to deal with. I had to contact them 3 times before they decided to authorize the return. They have 100% feedback but not many transactions.