Warning: Mollie is only available for countries that use Euro. Their main focus is on The Netherlands though, specifically iDeal and CreditCard.
If you want to use Mollie as a payment provider to charge your clients, make sure you create an account with them first through https://www.mollie.com/. This can be done for free.
Once you have an account on Mollie, you will need to create a website profile first. You do this by going to Website Profiles in the menu on the left (under My Account) and clicking on "create new website profile". Follow the steps and once you've create the profile, it'll show up under the same tab.
There, you will see a an API key at the bottom. Copy that API key and go to your portal on Virtuagym.
Start with your payment
On Virtuagym, go to your Business Settings > Payments. Here, you enable Mollie by ticking the box. Paste the API key here. Make sure you choose the available payment methods per payment provider. If you want to use PayPal through PayPal directly, check out this article on PayPal integration.
Now you can go to your financial app, find any invoice and under options at the top right of your screen, click on "email including pay-link". This will send the invoice as a PDF to the client, with the link to Mollie to immediately pay the outstanding amount through the preferred payment method. If you'd like to set up your own email, you can get the payment URL in the invoice under "Pay".
If you want to email more than 1 invoice to clients, you can select multiple invoices from the invoice overview. Select all the invoices, click top left on options, and click on "email including pay-link". Now, all invoices including the payment link will be sent to your clients.