For fields that are optional, make that clear by adding (Optional) after the field. Such as Phone Number (Optional) appearing in the default text.
Change default amount to $10, $25, $50, $100 and make $25 the default. Also, add some more space between the amount field and name so it's obvious that the range of numbers and amount field is clustered. For the Monthly donation, change to $5, $10, $25, $50.
Add physical mailing address as collected field (for end of year thank you where appropriate). Might want some verbage that we will send occasional mailings such as end-of-year thank you as well as potential other updates.
A few minor tweaks on the donation page:
Phone Number (Optional)appearing in the default text.Monthlydonation, change to $5, $10, $25, $50.