"The seller has agreed to issue a full refund for $52.46 USD. To receive this refund, you’ll need to ship the item (s) back to the seller by May 16"Does this mean I have to ship it by May 16 or does it mean he has to receive it by May 16?
If you can't confirm then make sure the seller receives it by May 16. If you can. It's what's known as playing safe. Unless you want to take the risk for some reason.
It stands to reason that he means it must be in his possession and undamaged by May 16 if you want a refund.
The way I read it - you have to ship it by the 16th. However, that may not be what they intend to be saying...
The key words in that sentence are “ back to the seller by May 16th”, so in his hands on or before that day. Make sure the post office knows that and they will give you different rates on how fast you can have that package delivered.