Access the Checkout menu

In the upper right corner you will see a [1 X] button, it allows you to cast large amounts at once

Tap it and enter the amount you want to post

The button will appear in black with the amount you entered in the previous step

Choose the product you want to add that amount to your cart and it will appear on the green button at the end of the screen

Continue with your sale as usual

Access the Checkout menu

Choose the product you want to add to the cart

Add quantities by clicking on the + sign at the top of the image

In the order listing, enter the quantity in the field to the left of the product name

Continue with your sale as usual