Being a self professed addict to these, I can assure you, with the utmost confidence, from all of my experience corresponding with over 50 current and former PS via this system... It all depends. My data shows over 50% never even acknowledge the gifts (and I do as much as I can to assure they are real and not someone posing on Amazon or even Amazon AND Twitter... an obvious pitfall since Amazon hides the names and addresses (although some of the secondary vendors didn't hide the info, as recently as a year ago and forwarded the delivery name and address (that has now been remedied, as I have not seen it in months))), but I digress).  

I tend to reward those who do respond with follow up gifts.. of those I would say about 10% seem open to meeting for real. Of that 10%, the overwhelming majority (90+%) are either currently (or formerly) known to provide. In fact many who don't respond at all are already known to provide. Basically, those who aren't already on one of the major sites looking to provide aren't thinking of starting because you send them something from afar without meeting you.  

I will tell you that after meeting them that can change... but you have to find a way to meet them in person after you send something, and have sent them something they remember and not something typical (like a bra or panties). If you are trying to arrange something from the other end of a keyboard expect crickets.