By default, NationBuilder displays an "Other Amount" button for supporters to donate a different amount than what you may have listed. To hide the "Other Amount" button add this tag to the page:
donation_hide_other
Adding a link to your donation page to an email blast or on social media? Add the following text at the end, replacing 25 with the amount you'd like people to donate:
?amount=25
Example: http://yourdomain.ca/donate?amount=50