Below is an explanation of how to create three donation buttons with
fixed amounts. Then after pasting the HTML code into your web page,
you switch their button image with your own t-shirt image.
They actually click on the T-Shirt image ... which takes them to PayPal.
Once at PayPal, they don't enter in a value because the link already contains
all of the correct fixed amount.
You go to PayPal and click on Creating a Donation Button.
Enter a description and item number.
Click the radio button for "specific amount". (Enter that amount).
Instead of using the email address, click on the "secure merchant account ID" radio button.
Let it generate the HTML (also it saves the button).
Copy the HTML to your website.
Change the image reference in that code from their button image to your t-shirt graphic image.
On your PayPal Account ...
The button gets saved. There's a link to "view saved buttons".
View it (to make changes) and click on "create similar buttons" to make
two more buttons for the other shirts.
Here is an example ... view the HTML code.