Cash donations are not deductible unless you have a receipt from the donor. That's in black and white on page 19-20 here. https://www.irs.gov/pub/irs-pdf/p526.pdf
That's not to say you can't list them, but if you do, and if you are audited, you can expect to lose those deductions.
I don't think anyone here has any insider knowledge on what triggers an audit (or a lower level IRS letter). I got a letter one year about my donations (not a full audit, just a letter asking to see proof of the donations), but since I had filed an amended return, I explained that, and they never wrote back to ask about the amended return. I suppose my file was selected randomly and my response allowed them to close it out in the system, and the system did not select my amended return. Who knows?
