Portals are portals, to move information more securely than other options back and forth. To get verified signatures that the IRS accepts on forms such as 8879, you need something designed to the IRS specs for signatures. What all are you trying to do in one software?
eSign will deliver verified signatures and it does deliver the tax returns to your clients. You can use it, at a lesser fee, to collect other signatures without verification, such as your engagement letter. But, it won't work for your clients to send their tax documents to you.
FileShare will let you upload any documents to your clients, including signature pages. It will let your clients upload any documents to you. It does not verify signatures.