When free postage is offered and selected, by you, I take a video of the stamped and addressed envelope (close up), disappearing into a mail box. 
That will be my proof of postage.
If you message me, I will send pics of the items requested for your approval.
Otherwise it will be a random choice.
Happy to combine postage with other items purchased.
Message for price.

If purchasing more than 1 item, unless specified in listing, ask for a invoice total. I can combine postage and possible provide a discount on undiscounted items.