Send in to repair through post box

I have requested a repair for my Iphone. I received the empty box and have managed to send back to Apple. But due to Covid-19 the post office (Royal Mail) nearest me is closed. I decided put the parcel in post box instead as I have read that it's a free postage (paid by recipient). But now I am very worried that my Iphone is going to get lost as I don't have any proof of postage because I just put the parcel in the postbox. Can anyone advice me on this please? Should I call the post office or Apple to at least ensure myself that my phone is going to be OK?