It isn't the cost of the stamp that's the issue; it's the difficulty in buying them if you are homebound and do not have a credit card. You can't order them from the postal service without a credit card, and most grocery stores have stopped selling them now (or if they do still sell them, require buying them in person.) Many people have difficulty getting to the Post Office during business hours if they have to depend on getting a ride from someone who works during those hours.
Sure, it's not impossible, but it's really unnecessarily difficult when the whole point is to allow people to vote without having to leave the safety of their homes.
Yes, you can go in person to the Board of Elections office to drop it off, but it's in a downtown area that requires you to pay to park, and the hours are very short. Again, not impossible, but difficult enough that it's obvious that someone is deliberately throwing up obstacles to discourage potential voters from making the effort.
All St. Louis area public library systems are hosting volunteer notaries, but their hours are limited due to the pandemic, so again, not very convenient.