1) Create a new payment form (or find the form you would like to edit.)
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2) Find the Amount section and choose Decided at Checkout.

3) Click Yes to the question "Add suggested amounts?"

4) Enter the Amount you'd like to suggest.

5) Click the Add Amount button if you'd like to add another amount. Enter the additional suggested amount.

6) Repeat step 5) for any other amount's you'd like to suggest
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7) Once you've finished your suggestions, click the Save & Finish or Save button.

8) Your form will now allow payers to choose between one of your suggested amounts or to enter their own amount when they go through the checkout process.