What Andy sais is all true, just want to add a small note.
I know in some countries it was possible for people to get customised stamps from their official post office. Customers could bring images for the stamps and receive a few sheets. Thise stamps may be used on letters and packages.
This means they are valid post stamps and unique in their own way. I haven't seen these stamps and I have no clue if they are something like the customised stamps.
But it doesn't make it official merchandise and it depends on how far your collecting hobby goes
