You can add a link to your PayPal account either in the instructions for your organization’s home page, instructions on an event signup page, or as a custom prompt in the volunteer signup screen.  However, ivolunteer.com does not currently tightly integrate with PayPal in such a way to guarantee that a payment is made as part of the volunteer signup process.  You can at least use the Custom Prompt method to ask the volunteer to check a box indicating that they have made a payment.  We do plan to have tighter payment integration in the future.
To add a link to your PayPal account in the event instructions, administer the relevant event, go to the Signup Sheet Design tab and press Instructions. Highlight the relevant text and press the Edit or Add Link icon. Press Save when you are done.
To add a link to PayPal as a Custom Prompt, go to the Settings tab | Prompts sub-tab.  Press Add Prompt.  Select a relevant Type, for example CHECKBOX.  Make the label “I have paid,” make it required, and make the short description “Click here to go to PayPal to pay,” and check the Hyperlink box.  A dialogue will appear where you can enter the link to PayPal.  Press Save when you are done.  By default this custom prompt will appear on the signup screen on all of your tasks.  You can selectively remove it form any task under the task’s More Task Options.