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.

__________________
What?

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.

This post is to explain what eventually happened so that future readers can see the full picture.

As the tax office advised me I sent a tax complaint via registered mail. About one month later I received a letter from the tax office in which they acknowledged the fact I send them reclamation but with no further explanations. So, I didn't know neither when there will be a new estimation nor what I should do in the meantime. At the day of the dead-line I paid 105 CHF which is the amount of the tax I actually have to pay (according to my calculations).

About one month later a new letter came with new calculations. This time the tax at the source was included as well as the payment of 105 CHF that I made in the meantime. I was charged with additional 40 CHF for some kind of interest that I didn't understand. I took it as a kind of fee I have to pay for the second review, I paid these extra 40 CHF and all was settled. Now everyone is happy