If you’re asking for donations, you can easily use Gravity forms. You can set amounts yourself or have the reader choose their own amount.
Here’s an example of a form with explicit amounts plus the ability for the reader to make a donation with an amount of their own choice.
Start by making a basic Gravity form
Add a Product field and set it up by adding preconfigured amounts to donate
Add another Product field – this time, in the dropdown box, choose User Defined Price
Save your form. Make a Page, add the form
If you already have a Donate form, simply edit that form to add another product field. Save the form. You don’t have to do anything else.