Shipping cost is not the only problem for you, sometimes the products won't get approved by customs if its send by someone personal on particular country so you need to analyse the receiving country's laws as well before making an estimation because he might say that he didn't receive anything so asking for payback from escrow. If everything clear on the receiver's country then you could ask for $50 for shipment in total and return back if there is any left amount on that $50.
Imo the approval is less of a problem for them unless the product that he's buying for someone is something unusual like guns or other weapons. Without knowing the country of the receiver and size of the product it's hard to say that $50 could be enough for the shipping cost.
What the deposit for, by the way?
I'm guessing the deposit is for the currency exchange part(to speed up the process) since OP mentioned the possible delays could go up to a day and wanted to reduce the fluctuations as much as possible.