I go to my address, and I get 3 domestic letters, and 2 international letters (as an example). I have two choices. One I could go to the post office, and get the stamps I need, or two, I could use the metering service, and fund that service plus pay for the convenience fee.
If I choose to get my postage at the post office, I will have to figure out what weight the letters are, make note of that (if over 1oz), then go to the post office, wait in line (dealing with crowds) and buy the appropriate postage. I will have to make note what is international, what is for mail over 1oz, and what is regular stamps. I will then affix the postage on the letter, and drop the mail off. Next time I have to send mail, I will have to do this all over again. With that said, it is more inconvenient to get my postage at the post office.
The second option is a meter service. I am paying mostly for the convenience and professionalism that I would have by using a metering service. This service is not cheap. It is $20/month + postage fees. I do get a discount on domestic first-class mail, but this discount is not nearly enough to cover the $20 monthly fee. For domestic mail, everything is printed on the envelope. For international mail, post cards, or large envelopes – I will print the postage on Netstamps, and use that as if it was a regular postage stamp (with exact amount). The postage is calculated, and exact postage is printed on the envelope or netstamp. I can then drop mail off the next time I check my mailbox.