Patio11 was ultimately asking for the owner of the free software project to emit an invoice, which has its own financial bureaucratic burden in which the owner most probably won't want to enter. Ie. it won't work because the typical free software developer doesn't want to incur into the taxes that this could imply.
For example, in Spain you aren't allowed to extend invoices if you aren't first registered as an autonomous worker (freelancer), which most people aren't because they already work for some company. But even if you did, then you'd have to pay for the obligatory social security for freelancers, which is around 260 Euro per month (so staying registered indefinitely as freelancer is not financially sensible, if you are not really doing actual contract jobs).
I would think this is similar in most other European countries, which in other words means that for the most common of the FOSS developers, writing a legally valid invoice is a no-no.
Of course the matter changes if the company doing the donations doesn't really care about the invoice being legally correct inside the country of the developer. But I wouldn't advise the developer to do things like that... (s)he would be risking a fine.
For example, in Spain you aren't allowed to extend invoices if you aren't first registered as an autonomous worker (freelancer), which most people aren't because they already work for some company. But even if you did, then you'd have to pay for the obligatory social security for freelancers, which is around 260 Euro per month (so staying registered indefinitely as freelancer is not financially sensible, if you are not really doing actual contract jobs).
I would think this is similar in most other European countries, which in other words means that for the most common of the FOSS developers, writing a legally valid invoice is a no-no.
Of course the matter changes if the company doing the donations doesn't really care about the invoice being legally correct inside the country of the developer. But I wouldn't advise the developer to do things like that... (s)he would be risking a fine.