Don’t redirect to the cart

This is actually one of Google’s best practices for mobile retail. If generally your visitors only buy a single product, I understand the temptation to redirect them directly to the cart. However, that breaks their expectation of an Add to Cart button.

One solution is to switch the call to action button to a “Buy” button. Otherwise, I recommend keeping it to a regular Add to Cart button which simply adds the product to cart. Let them open their cart if they want to checkout.

A modal panel is a more elegant solution

If you have lots of single-item buyers, you can display a modal panel when a customer adds a product to their cart. A modal panel is an overlay window that prompts them to check out or continue shopping.

The modal panel should:

Occupy no more than 50% of the display window.
Be only shown on user prompts. Don’t show modal panels as pop-ups.
Be easy to dismiss.

Modal panels should be avoided on mobile design though. Due to the limited space, they either turn out to be too small or too large, covering content. The cancel or close button may also be hard to click on.