Create a membership form with extra donation using Paypal

I want users of my membership form to have the ability to pay for one of two membership options and also add an additional donation if they choose. Is this possible? For example:

membership

1 year = $15

5 year = $75

would you like to make an additional donation?

$25

$50

$100

other amount

Also, can I upgrade my membership for a month and then switch back to free? During most months I will not have more than 10 PayPal transactions but during conferences and recruiting events I may have more than that. 

That's possible and there are 2 ways to achieve that.

1. You can achieve this on the Paypal payment tool alone.

Demo: https://form.jotform.com/70587314943967

You can clone this form so you can inspect the payment field settings and its products. Guide: http://www.jotform.com/help/27-How-to-Clone-an-Existing-Form

2. Or you can use regular checkbox fields and calculate the total using the form calculate widget. After that, pass the calculation field to the Paypal field for payment.

Yes, you can subscribe or cancel the subscription anytime you want.

If you need further assistance with this, let us know.