Pay invoices online throughout PayPal!
The PayPal Gateway add-on will allow your users/clients to pay the invoices they are assigned to via PayPal.
It allows you to accept payment through PayPal Express Checkout. When checking out with PayPal Express, the user experience is very similar to PayPal standard, except it is more secure. Your users will be redirected to the PayPal website for paying.
Extending PayPal gateway add-on
If the included payment icons packs are not enough for your, you can check out our Design Extras add-on. It adds several new templates or customizations that could maybe suit your needs, or at least show you how you can add your own icons packs to WP Customer Area.