You can send anonymous users directly to www.yourdomain.com/myaccount/invoices to direct them to the page seen above. Once your users have gotten to this page, they can enter the appropriate invoice number and billing zip code to find the particular invoice they'd like to pay. 

Once your user enters the appropriate information, they can either choose to only view the invoice or go directly to the payment screen.

If your user decides to only view the invoice, they will see the normal Invoice Details page, which also includes a link to pay the invoice, as shown below.

Whichever path the user takes to get there, they will ultimately end up on the payment page. This page will show them the subtotal of the invoice, any taxes or fees that are added on, and then the total amount due. The user can elect to pay a partial amount of the invoice, or pay the full amount, and decide whether to pay with a Credit Card or E-Check, so long as your site allows both options. If not, only the valid option will be shown. The payment screen is pictured below.