First, the issue with the link was on WIX side and it’s now resolved.
Per your question, let me explain the difference between “choices” and “customTextField” under CartIcon - Velo API Reference - Wix.com
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Options - These refer to “Product Options” under the product page in store manager. For example, the product “Create Your Own Canvas” has one options - “FRAMED SIZES” which choices are “SIZE 1” and “SIZE 2”.
When it comes to the addToCartAPI, you have to send all the defined choices with allowed values.
So - You add to cart is failing because you are sending the choice “Background Image” which is not defined and you are not sending the option “FRAMES SIZES” which is defined. And also the syntax is wrong anyway, for choices it would be “Background Image”: "$w(‘#print’).src
However, probably you don’t want to use options at all, keep reading. -
CustomTextField - This would refer to the “Custom Text” section under the product page in store manager. For the “Create Your Own Canvas” you haven’t defined any. The cool things about CustomTextField, is that you can use it to send any values you want during addToCart and you don’t have to define them in store manager at all.
So - What you probably should be doing, is sending the background image as a custom text field and then you should be good
Thanks,
Oded