As a vendor, why do I have to pay a monthly fee?

There are many ancillary fees that go into running a marketplace. Whether you are selling stickers on eBay or selling beer/cider/mead on Bevv, there is a cost of doing business. Bevv's economies of scale bring those rates down but can't delete them completely. 

Just a few examples are your smart pickups via UPS. UPS pickups aren't free. 

Running a tax calculation engine that actually calculates both alcohol and sales taxes where applicable isn't free. 

Hosting images and infrastructure on a cloud server and other technology integrations cost money and upkeep. 

Grab a beer and have a thought of all the costs of doing business that you would encounter from the logistics, legal, tech, and infrastructure side of running an online direct-to-consumer craft alcohol storefront.

Exactly...  Hopefully, that makes more sense.