'guy'/'guys' is used without a gender context in certain regions, more like synonyms of 'folks '. I hear "hey guys" routinely in SoCal said without intention of labeling people.

Only in the plural form though, right? I'm in the UK, where the usage you describe is common too, but reading "this guy" like in the comment above, I would assume they're talking about a man.

Your understanding is common in the US as well. Singular “guy” would read as “man” or “boy” while the plural “you guys” can read as gender neutral (though some argue that “you guys” should be avoided in mixed gender groups for being inadequately inclusive). I would expect there are some regional US differences in usage and reactions.

Even then it's highly context-dependent. Ask any heterosexual man "how many guys have you kissed in your life?" and not a single one of them is going to consider this instance of "guys" as encompassing women.

Another red flag: this invoice was issued just days before the EURO was introduced in Germany (1st of January 2022). Invoices issued in these times would have surely shown the amount in EUR as well.