As soon as it goes over 50gm, even 55gm, it goes up dramatically.
the trick is to not do these things over the counter at the post office, buy a box of pre paid registered post stickers and a couple of boxes of stamps and just do it all at home and put your stuff in your local mailbox yourself
ive never had any mail come back to me for underpayment, as long as you dont abuse it too much like putting $2 of stamps on a 5kg item, nothing will ever happen