"Buy now" button

Friends, the second month is coming to an end, as I am developing a payment module for FlentKart in the RU zone. There are enough problems.

Now we are faced with the quick purchase feature. If you click on the "Buy now" button, then everything that was in the shopping cart disappears. There's only one product left that we need to buy NOW.

Is this how it should work? Or are we doing something wrong in the program code?

Thanks for the support response and maybe someone has already encountered a similar problem.

How can I correctly add a product to the cart using JavaScript without removing the items that are already there? Currently, when I click the ‘Buy Now’ button, the existing products are removed from the cart.

Thanks!