Yes, you can give more than the amount set for a reward.
Once you have selected your reward, you can add a free donation to your contribution. Scroll to the bottom of the page to find the “Support with a donation” option. Enter the amount you want, then click “Add”.
The donation then appears in your summary, and its amount is added to the total of your contribution.
You can also add more copies of the same reward, if available, using the “+” and “-” buttons. The cost of the rewards and the total amount are updated accordingly.
Finally, you can also add different rewards. Scroll down the page or click “Add a reward”: the list of available rewards appears. Click the one you want and it is added to your selection automatically.
When you're done, click the “Continue” button below the total to go to the payment page.