Add pay what you can option to buy now button options

[AliaDoula]

I am setting up a gift certificate buy now button for my website with a few options. e.g. 1 hour $50, 2 hours $100. 

I would like to have one option be an "other" where the buyer can type how much they want to gift.

Can this be done with the buy now button?

Is there a different tool that would work for this purpose of both having fixed price items and one flexible item?

See here as an example:

https://app.moonclerk.com/pay/7dura17ej7el

Thank you!

Alia