An add-on with sufix COM (COMMERCIAL) is paid version of add-on. Sufix UA (like Ukraine) denotes free add-on.
How it goes with payment? First you make $5 donation (you can pay with credit card or via PaylPal) and for this $5 you will get 225 days of grace period for free access to all free add-ons (without limitation of 2 downloads per 3 hours).
Now, for each commercial payed add-on you have to give (pay) 85 days of this grace period. So if you want to buy commercial (last version) of add-on, your grace period will be reduced for 85 days. If you take another commercial add-on, it costs additional 85 day until you exceed your grace period. Then you have to make additional donation and it goes all over again.
But bear in mind, this payment (donation) is given just for packaging of components into NAS add-on. With it you pay just effort of packaging the add-on itself. If software that is packaged inside add-on (majority of them are) is freeware you will not have to pay anything else. But if packaged software itself is commercial one (e.g. Twonky media server) you will have to buy license for this software from original company which made this software.
What's the difference between commercial and free add-on, well, commercial version packages the last version of components and software which means that has bug fixed and additional functionalites. E.g. with free add-on of Transmission BT client you'll get basic functionality and basic WebUI, commercial version will give you web interface to change configuration and new Web interface.