Deductions & credits

I actually have this same question. I do have a legitimate 501c3 animal welfare organization, however we are new and do not have a donor base yet, so all treatment and care (food, medical, etc.,) for the animals has been out of my own pocket. One question I can’t seem to find an answer to is whether I need a letter from my own not profit as I would give any other donor? 
Also, are credit card payments made to the nonprofit (vet bills for example) deductible or just what has been paid toward the credit card for that year? 

thanks so much!!!