Used Stamp Collections

Do you tear off the used stamps from correspondence you receive, collect them and then don't know what to do with them?
We have the name of a contact who will value and buy them from us. We do need a lot of stamps though, so if you can help please weigh them and send in a correctly stamped envelope addressed thus:

For the attention of Val
Northampton Cats Protection,
PO Box 5522,
If you have a large amount please call the helpline, and we will advise you where to deliver them, or arrange to collect them from you. 
Thank you!