multiple membership contact form with donation and PayPal - can this be done?

I'm looking for a WP plugin that will meet our organisation's membership collection needs. Here is the scenario:

We need to collect membership fees and donations from our association members. In single transaction the user must be able to purchase multiple memberships (eg. husband pays for himself and his wife in a single transaction). The user must enter the contact information for each individual member that is being paid for. Memberships have a fixed price.
We also want to have a donation option available to the user when they are purchasing the membership(s). We prefer that the donation value be set by the user to whatever value they would like to donate. The form software would then total the cost of the memberships and the donation. A PayPal Buy button is then used allow the user to make the payment.

Can Gravity Forms software be used to build this solution?

Thanks for your help.

Victor

Thanks Carl,
Yes, by membership I mean membership dues for our organisation. We do not have any membership structure for our web site - anyone can visit the site without restriction.

If I understand you correctly we could sum up the membership dues in the transaction, and then use a "donation" product type to add the donation to the sum, forming a single transaction with a single total cost. Could we give the user the freedom to specify the value of the donation, or would we have to have different donation levels specified?

I should point out that the reason to include the donation within the membership dues transaction is simply to make it easier and quicker for the user. User's won't have to fill out another form and go through a separate PayPal transaction to make a donation, which I think is the only other option.