In some cities, they charge you a yearly fee to register to be able to get permits. In Fort Worth, I believe this is $120. (Hopefully, this is cheaper where you are at.) Another town I have signs in does not charge any type of registration fee.
Without the registration, you can’t pull a permit for a rental sign. So, it basically forces customers to rent signs and not purchase them. If they purchase signs (in Fort Worth for instance), they could only put their sign out every other month. However, there does seem to be a gray area when the sign is for a non-profit. (Apartment complexes also seem to have some different rules.) Regardless of anything I am writing here, it is VERY beneficial for you to pick the brain of someone who knows how sign rentals work in your local area.
So far, it appears the applications can be done via email. Typically, their is an application and some type of map needed (Google maps and a graphic program) should set you up pretty well. As long as you have a scanner/fax, you should be able to get most of this process done. You may need to pay the fee via a website OR they will charge your credit card directly. If all seems to work out. (There was the time I received an email where I was charged for an unaccounted for amount. It turned out to be a rather unsophisticated copy/paste with an email address-I got someone else’s email.)
If required, the permits are just a necessary evil. If the customer’s have rented from someone else before, they know how it works. You provide them an invoice with your sign rental charge and the sign permit fee on a separate line. If you are going to do this, the paperwork is a part of the process!