We send out a couple of mail bags each day containing small packets, some UK, some air mail, and have an online business account with Royal Mail.
We print out our own stamps and stick them on each packet.
At the end of the day, each mail bag is weighed and the weight divided by the number of packets to get an average weight.
Go online, create an order and print it off. Our mail is collected each day but you could just go to the post office and dump the mail bags there without queueing.
OR/
You could rent a franking machine and print off your own stamps after weighing each packet. Put them in a mail bag and again dump them at your nearest post office without queueing.
This is what we used to do before our workload increased.
We find the first option the best and most convenient