One of the main pain of upselling bundles is when it comes to keep the original SKU of each product that is part of the bundle, in order to be compatible with any 3rd party fulfillment system like Oberlo, inventory/accounting system like Quickbooks or shipping management tool like Shipstation.
We finally solved this pain. Here's how below.
Once you install the Ultimate Upsell app, create a new upsell and scroll down to Upsell type. Then select the last option called Bundle (each item keeps its own SKU).
When customers buy the upsell, you will see the SKU of each item in the order. In other words, this new bundle type doesn't create a new bundle product in the background. It takes the original products that you want to upsell and offer them as a bundle.
This means that when the order is passed to your fulfillment system, it gets the right SKU of each product purchased inside the bundle. This also means that your fulfillment system can fulfill the order like a charm.