Showing instock quantity in check out

I have an online store that goes "live" at a particular time.  Customers are notified of this time and items are scheduled.  Sales all happen very quickly.

The issue I have is as follows:

• Product Z has total inventory of 10
• Customer A adds 10 items of Product Z to their cart, continues to shop adding more products of X Y etc to their cart.
• Customer B adds 5 of Product Z to their cart and checks out before Customer A. 
• Customer A then proceeds to cart, but Product Z does not show an inventory change until they proceed to payment.

I would like to see if its possible for Customer A to see this when they proceed to check out, rather than once they have completed payment details etc?

thank you

Thanks for your response Paul, much appreciated.  Is it possible to show stock levels to the customer at the check out, rather than just a message below?

Examples I have seen on other websites are "there is only 3 available of this item" etc.  This would allow the customer to quickly change the QTY before submitting payment.

4 hours ago, hconnor said:

Is it possible to show stock levels to the customer at the check out, rather than just a message below?

No, it is not possible to customise the checkout page in any way. This page is locked down by Squarespace to prevent code from affecting the secure payments process. Customers can only enter alternative quantities by trial and error to see how many are available.

To make a feature request, please contact Squarespace Customer Care using a support ticket.