Enabling this mode means that now the cart will include products attached to options via SKU as separate products, in addition to the custom one.
Enabling Bundle Builder Mode
First go to product Settings
Then switch to SKU Management tab and enable Bundle Builder mode switcher
Connecting existing Product to the Option
First, you must open an existing product in Shopify, scroll down to the list of variants, open one of the variants, find the Inventory section, and copy or enter a new SKU of the product (variant).
Then look in the Custom Product for the option to which you want to attach your existing Product.
In the options editor scroll down to the SKU field, enter a value there, and click the "Connect to Existing Inventory" button.
If the connection is established, the button will look like this:
If the SKU is out of stock it would stop selling, unless the checkbox "Continue selling when sold out" is not checked. If items in inventory are not enough for the purchase (ex: inventory is 5 and customer orders 6) the pop-up will show up explaining that there are not enough items in the inventory for the order.
Setting the number of product units in an option
Here, in the option editor, you can set the quantity of existing Product that will be added when the buyer chooses this option. Please note that the option price in this case will be multiplied by this amount.
Choosing the option in this example will increase the price of the Custom Product by $400
Setting the amount of added existing product through the category
If you don't want to specify the quantity of each option, you can do so for the entire category. To do this, go to the editor of this category and scroll to the Quantity Multiplier Source and Quantity Multiplier Value fields.
If you want the buyer to be able to choose the quantity of the added product himself, then as the Quantity Multiplier Source you can select one of the existing categories in this tab.
Note that multipliers for quantities from option and category are multiplied. You can enter 2 in the category editor, and 4 in the option editor - in this case, the total number of existing product for this option will be 8.
Don't forget to save your product!
How this example will work for a customer
Base product price is 10
Then the buyer selects the Race option, which costs 100, but the quantity in the option is 4. Price increased to 410.
The customer then selected a multiplier for this set of 3. The price increased to 1210 and the number of products attached to the Race option was 12.
After clicking the Add to Cart button and moving to the cart, we will see that along with the Custom Product, another one was added, the price of which is 100, but since its quantity was chosen 12, its total price increased to 1200, and the Custom Product was added to cart after deducting this price.