Typically I email invoices to clients but we also have the option to click a link on our website that opens a pre-filled invoice and they can easily pay rather than asking for an invoice. I set these up years ago --but rates have gone up and I can't remember how I created these invoices. We are not using any graphics or buttons from Paypal.... I simply linked text which opens a prefilled paypal invoice.
When you click on the link for the invoice - where does it take you - is the custom invoice on your server, in your web site, etc. Sounds like you built a custom form that has a pay now link to PayPal which is stored somewhere.
