You can provide one-click upsell offers within a membership to clients who are logged in. Here's how it works:

• Create separate memberships for each offer. 
• Create a membership group for the offers.  The initial offer purchased, and the upgrades, and any additional offers for which you want a one-click upgrade from the initial offer, must be in the same Membership Group together. You can create a membership group under Manage > Membership Groups
• Enable the option to "Allow one-click upgrades/downgrades when logged in to member's area".
  
• If you want to offer multiple offers and/or don't want previous memberships cancelled on upgrade/downgrade within the membership group, select the option "Do Not Cancel Previous Membership Whenever A Member Upgrades/Downgrades". (By default the behavior is to auto-cancel the previous subscription on upgrade/downgrade)
• Add each membership to the membership group from the Edit Listing page of each Membership, in the "Membership Grouping for Multilevel Memberships" section.
• Copy the embedded form code for each offer and place the code in an episode in the initial membership. You can set the episode to be default, if desired, so that the episode with the upsell offers are the first thing seen upon login to the offer. (You can get the code under Create > Forms, Buttons & Links).
  

Currently, an offer can only be in one membership group at a time, so you must plan your membership groups accordingly, or make a mirror/copy of an offer if you must have it in multiple groups.

Also note that if you request additional form fields in a subsequent offer that you did not initially request in the original offer (such as shipping address for physical fulfillment) the system WILL ask for that data, so in that condition, it would not be true one-click. However, it will only request those additional fields.