How can I return a cell phone with charged battery to Amazon?

Here's the thing, I live in Peru and I purchased an iPhone 7 Plus by mistake due to a misunderstanding. I realized that days after doing it so I immediately canceled the order. It had already been shipped to my address and when DHL contacted me, I told them to return it because the order had been canceled. It's been like a week since that(The package was on hold) and now DHL says that the item can't be sent back because the battery is charged so they want to give me the cell phone to discharge the battery and then pick it up to ship it back to the seller. Discharging the battery implicates opening the product which I can't since the seller's policy says that They won't refund opened items. SO, it's really kind of a big deal, any suggestions? What should I do?

Let the battery run down