customers send you a bitcoin transaction printed on paper. 

You scan the transaction and broadcast it to the network.  

Your fee could be the greater of some minimum or like 0.01% of the transaction value.  

The fee could be sent as just another paper transaction, it could even just be cash.  

You could expand into a snailmail based email or publishing platform.

Customers send you a letter.

You make the letter available in one or more of the following forms: 

1. as a single webpage

2. sent via email to a specified address

3. in a data dump along with all open letters received that day / week / month / year / or decade.