i've searched for a solution (or similar) for the problem below, but so far without any luck.
I've a customer who needs an online store with few requirements
-> No stock control (Magento2 already provides this)
-> Hability for the customers to place orders (skipping the payment method or finishing with a "placeholder" method (some variation of Zero Subtotal Checkout) )
-> Transactional email of the order should have the order details (Magento2 standard) and also a link to a Payment Gateway so the customer can pay the order
-> After order payment, the status on the backend should be updated as well
Magento already provides some of this features, but not all and i wasnt able to find any way to achive the above.
WHich feature isn't provided by Magento?
I guess you're talking about including the payment button into the email, right?
Also the "payment page" where tthe customer only need to place the CC details.
In that case, from my point of view, that is a feature of your payment method and not a Magento feature.
In the past I did something like that for a Payment Gateway here in Argentina. It was with Magento 1 but the idea works. I created a new payment method (in my case, available only for backend) and the email sent to the user includes the button.
Then, the user, from the email, click on the button, is redirected to the Payment Gateway page, and pay. Finally, the customer is redirected to the Success page into the store.
That's what you're looking for, right?
This is the link to my old M1 module.
Maybe (just maybe) can help you to think about your implementation on M2: https://github.com/barbanet/magento-dc-mercadopagoplus