I would never bother listing small debts in my tax return but on Dec 31 I make sure I pay all open bills
I have a margin loan so during the rest of the year usually I pay my bills on the latest due date. The only exception is Dec 31. My marginal wealth tax rate is close to 1% so why not, it pays for a nice meal… 
That is what I would do. If possible I would avoid a cash transaction, so that paper trail is automatically created (makes it more legit).
I always make quick receipts (with the points mentioned above) when I buy stuff second-hand, and I include additionally the known defects. Very important for electronics in my view.
I also include a sentence that the sale can be revoked if defects are discovered afterwards (usually within 10 days). It sounds all very formal, I know, but never had any issues with trustworthy sellers to sign those receipts.
However, I for sure avoided many bad deals. Most scammers obviously don’t want to give their identity, would not sign, etc.
Going back to the original question, I don’t think a formal receipt is required for the tax office to accept it. A TWINT transaction, or a bank wire with the reason clearly stated in the message should be sufficient, as long as it is not too frequent… in which case it could raise suspicions.
I can now report that the “CHF 200 «Haustür- und Gassenspenden» under donations” was NOT accepted on my tax declaration in SG. The tax office rejected it citing that all donations must be evidenced and no lump sum deduction (Pauschalabzug) is allowed.
9 posts were merged into an existing topic: Tax declarations are fun