Recommendations on File Sharing Service or Approach

I'm going to be managing a software contract that involves a software rewrite.   I need to initially (soon) provide a large amount of RFP information to the prospective development vendors, followed at some point by the code.  I would like to do this using security that  is greater than email but less than public key encryption.

I'm thinking of getting DropBox, Box, SugarSync or similar service, with an access URL to the shared folder, and then a password to get into the shared folder (which will be communicated securely.)

These services will also be used for general back-and-forth file sharing.  It will only be for 6 months.   I don't want to use the syncing or backup functions of these services; just file sharing.   $10/month is not a problem.

Any recommendations about which service, or thoughts about this entire approach, would be appreciated.

Thanks!