Donation form with multiple user-specified amounts?

Hi,
I am very new to gravity forms, so please bear with me!
My rail trail nonprofit would like to have a combination donation-membership page which would include checkbox for membership ($40) but also extra donation fields with user-specified amount. I’d like to avoid the whole shopping cart layout and just have a Total on the page. I want users to be able to enter credit card info securely to pay the Total.
Here’s a rough outline:

Membership / Donations page
Become a Greenways member $40 X [ checkbox - membership is optional
Additional donation to Greenways $_______
Dedication or in Memoriam: (text box) _____________________________ _____________________________________________________________
Donate the Capital Campaign to build Rail Trail $
____
Total Contribution $__(total = Membership + Addl-donation + Donate-capital-campaign) _____

Credit card info
Card number etc, expiration date, CID, name zip code…

    SUBMIT 
    Payment 
      Button

I hope this is clear! Thank you for any advice.