The "Shop Pop-up" screen allows you to change an items properties once it is added to the "Shopping" list.  Long pressing on an item in any of the "Shop" tabs brings up this window.
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There are 2 separate pop up windows.  One, an edit window where you can make changes and a "Display Only" window to display data without data entry.

 

The normal "Edit" window that pops up has a  "Blue" background.  The "Display Only" pop-up window has a "Green" background.

 

The "Green" window will only display when the "Click Checkbox Only" option is selected in the system "Settings" screen.  When this option is checked, you must long press the Checkbox itself to bring up the "Edit" screen.
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The edit screen has 5 data entry fields.
"Note"     - where you can store any notes to an item.  These notes can be inherited from the "Product" itself.

 

"Quantity" - where you can set the quantity wanted.

 

"Price"    - where you can set the price of an item  This price can be inherited from the "Product" itself when first added.  Once in the "Shop" list, any changes made are only for that Shopping List.

 

"Taxable"  - a flag that you can toggle on and off.  If on, taxes will be calculated in the Shopping Cart.

 

"Coupon"   - a flag that you can toggle on and off as a reminder that you have a coupon.
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When set, the Coupon and Note fields cause push-pins to be displayed on the Shopping list.  The color of these pushpins quickly shows what values are set.

 

A Yellow pushpin denotes that an item has a note on it.
A Red    pushpin denotes that an item has the coupon flag set.
A Green  pushpin denotes that an item has both a coupon and note.
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In addition to the 5 fields that you can set, there are many additional information fields showing information from the "Product" screen.
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Tapping on any of the 4 "Price" fields will automatically change the items price to the tapped price.  This allows you to quickly change the price to a previous stored price.