Gift certificates can be purchased in increments of $5. For example if you wanted to send 2 people $20 each, you would simply purchase a quantity of 8. Giving you a total of $40.
Once purchased the total amount will be available in your account. You can then send any amount or portion of your total to 1 or more recepients by email.
See the Gift Certificate FAQ link under "Information" for more information on buying and using Gift Certificates.