If there was insufficient postage, you would have still received the seeds, but after having to pay the postage defecit and a 1 pound admin fee...
Items are returned to sender if there is no such address, the address is totally illegible, if the building is inaccessible.....
Items are only returned to sender if there is a return address.
If there is no return address, items are sent to a mail centre in Northern Ireland, where they are opened, to attempt to ascertain a return address.
If they still can't be returned they are kept for a period (I'm not sure how long) to allow for claims from customers.
The P.o KEEPS no record of mail that has been returned to sender, so would not be able to confirm to anyone it had been returned as such.
Monk...x