Every year that I worked here in Lugano (Ticino) this exact omission occurred and was always revised in the final bill after I notified them. It seems to be a glitch in the systems. Upsetting the first time, then just mildly annoying, but expected, in subsequent years. Now that I am retired, only a faint memory.

-Dr. M.

Are you sure? In Ticino, and did it perhaps change?

Canton Zug says explicitly [Pt. 16] that, while the subject taxed is the employee, the debtor is the employer. Bern as well. Same for Vaud, the employer is the debtor here as well (deepl.com produces very good translations, far better than google):
The gist of the above:
Who pays someone else is the debtor for the tax at source, usually that's the employer.

OP, that should mean even paying the 105.- is throwing money out the window. You have 30 days to raise a formal complaint. You must do so using snail mail. If time is the issue, perhaps consider bringing the letter yourself (remember getting receipt confirmed, e.g. on your copy).

1) You sent the complaint using registered mail? If not do it.

2) You sent the complaint to the correct address? On the bill it is stated to which address a complaint must be sent and also by when.

In canton Zurich there are two different addresses. One if you contest the calculation of the taxable income and wealth, and another if you contest how the tax bill was calculated. If in doubt sent the complaint a both addresses, both time using registered mail.

Also on the canton Zurich tax bill it is mentioned that if you complain about how taxable income and wealth was calculated you must still pay the bill. However there is no such note in case you contest only the bill itself.

__________________
"Okay, I just hope we don't wake up on Mars or something surrounded by millions of little squashy guys."

Thanks for the quotes, Max! I appreciate it. I didn't have the exact text but it is common sense to expect that it is the employer that is the debtor for the tax at source. Now I have the text too.

Yes, I sent a registered mail and have the proof it has been received by the tax office.

1) Yes, registered mail
2) Good point. I checked both addresses and they are the same.