Yes we do, in that we are the distribution method for the code authors to sell their add-ons. We are not modifying, sharing, or otherwise copying that code, though. For all intents and purposes we are nothing more than the distribution site that the author’s use to charge a fee for downloading. See these four FAQs from the GPL FAQs:
Does the GPL allow me to sell copies of the program for money?
Yes, the GPL allows everyone to do this. The right to sell copies is part of the definition of free software. Except in one special situation, there is no limit on what price you can charge. (The one exception is the required written offer to provide source code that must accompany binary-only release.)
Does the GPL allow me to charge a fee for downloading the program from my distribution site?
Yes. You can charge any fee you wish for distributing a copy of the program. Under GPLv2, if you distribute binaries by download, you must provide “equivalent access” to download the source—therefore, the fee to download source may not be greater than the fee to download the binary. If the binaries being distributed are licensed under the GPLv3, then you must offer equivalent access to the source code in the same way through the same place at no further charge.
Does the GPL allow me to require that anyone who receives the software must pay me a fee and/or notify me?
No. In fact, a requirement like that would make the program nonfree. If people have to pay when they get a copy of a program, or if they have to notify anyone in particular, then the program is not free. See the definition of free software.
The GPL is a free software license, and therefore it permits people to use and even redistribute the software without being required to pay anyone a fee for doing so.
You can charge people a fee to get a copy from you. You can’t require people to pay you when they get a copy from someone else .
If I distribute GPLed software for a fee, am I required to also make it available to the public without a charge?
No. However, if someone pays your fee and gets a copy, the GPL gives them the freedom to release it to the public, with or without a fee. For example, someone could pay your fee, and then put her copy on a web site for the general public.