I see a lot of vendors sell big quantities, from a few hundred grams to several kilos, which I assume that they are send inside boxes or big envelopes that don't really fit in a mailbox. Does that mean that they go to the post office when they sell bulk? It sounds really risky.
Or maybe it is only where I live that only small, letter-sized envelopes can be sent using a stamp and thrown in a mailbox?
In the USA anything under 13 ounces can be placed in a blue box, this is to prevent terrorism. Stamps are stamps are stamps, so any combination of any stamps can be used to pay any postage, as long as you include enough. One time I tried to buy $5.75 stamps and they gave me 5 x international holiday letter stamps valued at $1.15 each, everything arrived as expected.