first question:
did you update / install *any* plugin manually (i.e via ftp instead of using the wordpress plugin uploader or updater) ?

the problem that i’ve seen in the past is that people upload/update via ftp but as a different user, so the owner of the files is incorrect (especially when adding new files and even more so regarding .htaccess files)

if the permissions or owner of an .htaccess file is not correct the server will – more often than not – throw a 403 “to be safe”

so first of all i would check that the owner of all of your files is correct and “chown” if necessary (or simply “chown” recursively once you know what user is supposed to be the owner of all files

of course, you also want to check the permissions of files (plenty of results if you search for that )

have a look at the suggested permissions here
https://wordpress.org/support/article/changing-file-permissions/
they are somewhat different

you might also want to check if your server has mod_sec or some sort of other security stuff (firewall etc) running (which may or may not cause this, if it does check why it throws 403 in your case – might want to speak to your host though)

sorry, but i dont understand what a 403 has to do with any kind of minimum order
and i’d also like you to talk me through the issue you are describing on your site.
i want to see that

thanks for getting it to the bare bones – i’d also suggest you disable all additional other gateways (though i dont think it’s caused by those either, but makes things easier)

in any even though, what happens is this in all circumstances:

you put a few things in the cart,
lets say here https://pizza.taverne.at/speisekarte/pizza/ I am adding
3x Käse-Pizza
6x Margherita Pizza 21cm
1x Margherita Pizza Ø 31cm

and i can add / remove / alter as i like. so far so good

when i now go to the order page (https://pizza.taverne.at/orders/)
i can see all those things in the cart as expected
if i now try to change *any* quantity (up or down) it throws the 403
if i *delete* one of the items i can subsequently change things as needed
and do whatever i like after that it seems

this somehow looks to me like a db and/or cache and/or session issue

check this :
a) clear your cache
b) put something into cart
c) go to orderpage
d) STOP doing anything else
e) check the [prefix]_wppizza_orders table for a new entry (corresponding to the current time if you already have orders in there, alternatively truncate that table beforehand so it’s easier to see)
f)if there’s nothing in there, than there’s the issue – i.e a db entry has to be added when a user goes to the order page the first time
g) let me know and we’ll see if we can pin things down a bit further perhaps

can you also please post your wppizza->tools->system output here separately ? (make it “set as private” if you want)