@nsh ok, thanks.
I have experienced this, and I don't find it that shocking, to be honest.
The FNRS does that for local (i.e. Belgian) reviewers, when we join for the day discussing grants or fellowship applications. The payment is indeed optional (and quite small), and is subject to income taxes.
The time involved can be considerable for larger grants, or as a chair of such a panel, who needs to check all submitted reviews. These are international reviewers who travel to BE.
@cazencott @lgatto Yes. Either I'm paid, or I'm not. The fact that you're asking *me* if I should get paid is worrying.
In what world do you ask a service to sometimes that is valuable enough that you are ready to pay them, but also can straight-face say "okay but you have to ask for it". If it was e-commerce it would look like a weird dark pattern where they are low-key hoping for you not to notice the checkbox.
There might be a reason why reviewers are asked. In the case of the FNRS, my local funding body, it's straightforward, so there's no extra admin for me. But I had situations where I was sent an internal check which cost me money and time to cash. (Although I don't think/remember it was optional in that case)
There might also be taxing differences if it's a de facto payment, or some sort of optional honorarium.