Forwarded donations are not tax-deductible (in the US); That's a lie that's been spread around the internet. If you give a company money with the express purpose of them forwarding it to someone else (the company acts as a "collection agent"), it's not their income or donation.
So it would be actually financially _better_ for Valve to donate a portion of their revenue and state "we will donate x% of the price to yy", as THEN it would be tax-deductible for them
It's not better, because being revenue and donated away and tax-deductible all cancels out. It's as if they never saw the money, just like forwarding it.
And even if it was, all "tax deductible" would mean is that they wouldn't have to pay taxes on that money. Which, you know, they don't get to spend. So it's kind of defacto tax deductible in the same sense that my friend's income is "tax deductible" for me, I guess.
A lot of people online have convinced themselves that "tax deductible" means that the government would refund you that dollar amount. That's a "tax credit"... If forwarded donations were a tax credit, then yes, rounding up is giving the company "free" money! But you're not.
Such donations might even be tax-deductible revenue for Valve, so even the finance bros should love it.
Although I would prefer if Valve simply commits to a fixed percentage of its Steam fee to be donated...