In the Inquiry Catalog mode the cart widget shows the quote cart. In Appearance > Widgets it is listed as Shopping Cart. Add it to any of your registered widget areas. The output shows the quote cart button with the number of products added to the inquiry. You can set an optional widget title.
To keep the quote cart visible while visitors scroll the page, check Stick cart on top on scroll in the Cart Widget section of Products > Settings > General Settings > Quote Cart. You can also show the button anywhere with the [cart_button] shortcode. See Shopping Cart Widget for the Web Store mode.