How can we include a donation option in our current payment form?

We are selling tshirts for an ALS walk and we wanted to include an  option for additional donation amount that a person can add and then charge all in one transaction.

I only see sale and donation as two separate options.

 We are using PayPal as the payment method.

The workaround would be to add another product item and label it with "Donation" then put the amount as 1 USD. Then enable custom quantity box so to let anyone input a quantity. 

Then you can inject a custom CSS codes to hide the 1 USD price. Here's an example form: http://www.jotformpro.com/form/52352678248968. 

Add the item and get back to us so we can help you get the correct CSS codes to hide the price.

Thanks