Would you pay more for stamps from a charity? If you could get a receipt. Would you give your stamps to charity? If the answer to the question would you pay more. Then charity might be a way to go. I have applied to work at Oxfam. Recycling used postage stamps. Given I just saw a SCV 100k sell for 9k it was VF with PF cert. And if you watched any of Firby auctions you will see items going to charity. I dunno I think its a good idea. I am giving about 10k SCV Canada, US to charity. Hopefully I will get a good price. Spink has awesome prices but I don't think I will include pre 1940 XFNH. Anyhow just wondering.
As far as paying more for a stamp if bought from a charity, there would be no advantage. You cannot deduct the actual value for the stamp, only possible the amount to paid over its value. As far as giving stamps to a charity it is a good thing. The USPS issuses a yearly stamp catalog listing values of mint and used US stamps and that could be used to help figuring the value of the donation. (Check with your accountant)