Although the risk is small you should not put the satchels directly into the post box if you want to make sure you have Paypal seller protectio. If you lodge them over the counter they are scanned into the system however if put straight in the box they may not get scanned until they reach the buyer's delivery centre. Should a parcel go missing before it reaches that point then you have no evidence that it was actually posted, only that you paid for postage and printed the label.