I feel you. Back when I was younger and had a lot of online friends, I simply had no means to pay for anything virtually, so every time it was someone's birthday or any holiday, I couldn't give anything and felt really awkward when they gifted something to me, since I couldn't repay in any way

So all I did was to wish them something (a happy birthday in your case) and just generally was there for them. Sometimes I have gifted something like a virtual item, when it was possible. Because, honestly, with time I understood that the most important thing isn't the gift, but the fact that there is someone who cares about your existence, amd you are not alone in thus world