How do you set up a Donation item in SCA where customer can enter the amount on Webstore front end?

That's not an out-of-the-box feature and is generally a quite complicated customisation to implement.

Thanks @Steve Goldberg. Sitebuilder had a 'Variable amount' field on the webstore tab which worked nicely. Is there no way to use the same feature in SCA?

Not AFAIK

That's not standard Sitebuilder functionality as far as I'm aware (unless you're on a special edition of Netsuite). If you search help for "Accepting Charitable Donations" there's a scriptable cart example on how to implement something like this. It sounds like you may have this, or something similar in your account. Assuming that is the case, it's probably feasible to adapt SCA to follow the same principle.

Variable amount it´s actually out of the box in Site Builder but it´s not supported in SC & SCA