we would like to buy 2 WGs firebox to use as firewall and VPN site-to-site.
I'm wondering if, in addition to their price, I have to pay something for some licenses...
and if there is any kind of annual fee...
A new firewall normally comes with 1 or 3 year licenses, depending on which you buy.
After that, basic firewall functions work without any additional licenses.
Optional additional features do require ongoing licenses.
WatchGuard Security Serviceshttps://www.watchguard.com/wgrd-products/security-services
You don't HAVE TO buy them with any annual fee (after the first year), but it is HIGHLY RECOMMENDED that you at least get Live Security each year. This license gives you firmware updates and I believe warranty replacement.
Now in my use, I won't sell a Firebox without the Total Security UTM license because I want my clients to have the best protection possible.