A 'sender' is someone with a Dropbox Sign account login who uploads and signs documents or prepares documents to be signed by others, then collects them back via Dropbox Sign.
For example, if you send a signature request to a client via Dropbox Sign and that person simply signs the document, that person is an end-signer, not a Dropbox Sign sender. You are the sender in this case because you prepared, sent, and collected the signed doc via Dropbox Sign.
You do not need to pay for your end-signers as they are not considered senders.
