With Superbill you can now manage invoices and client billing directly within WordPress.

Superbill demo area:
https://supertest.freshlondon....
Username: testuser
Password: testuser

Automatic payments
Payments accepted via PayPal and Stripe (additional payment gateways can be added upon request).

Currency conversion
Supports full currency conversion, currently using PayPal's supported currencies:

AUD / CAD / CHF / CZK / DKK / EUR / GBP / HKD / ILS / MXN / NOK / NZD / PHP / PLN / RUB / SGD / SEK / THB / USD

Time your tasks
Use a task timer to add timed tasks to invoices.

Charge late fees
Charge interest on late invoices.

Late fees can also recur at specified periods, for example every 7 days after the invoice due date.

Add your own products
Create and manage products and easily add these to invoices.

TAX / VAT
Include TAX / VAT charges for specified clients

Mates rates
Specify which of your clients get a 'mates rates' discount, at the client level or individually per invoice.

Automatic emails
One click to send an email, letting a client know that their invoice is ready.

WordPress 4+
PHP 7+

Upload the plugin zip directly within the WordPress 'add plugins' page.