You can “Correct” an envelope and remove documents as long as the first recipient has not yet completed their signing. You could also make the signature tags conditional on a “cover page” with the options and radio buttons that would drive that conditional logic. Then the signature tags would only show on the document that they choose. All of the documents will still show in the final package, but unsigned. 

If radio buttons are used, then would the customer be able to choose only one document?

My question is: If I send my customer three proposals in one envelope is there a way for the customer to choose only one, two, all three, or none of the proposals? 

They could choose to sign only some of the documents, but you would have to use checkboxes instead of radio buttons if you want to give them the one, two, three or none options but the unselected documents would be in the final package. You could also get creative and have big fields in red that say something like “Unselected” conditional on those checkboxes being UNCHECKED, that way it is easy to see that one of the documents wasn’t chosen. If you did that, it would have the unselected text showing until they select it, and then the signature fields become available and required.