Keeping Google happy - attributes for a bundle

[cosmicray]

I have 8 LEGO sets, that I sell as a bundle. See this link …

https://cosmicray.ecrater.com/p/19398889/lego-3560-3561-3562-3563-3564-3565-3566-3567-nba-complete-set-of-8

Each of those 8 has it's own MPN and UPC. I wish to see them as a bundle. All but one of them, I also sell individually. That one is in such short supply, that I do not sell it individually.

Google says I need a UPC or MPN, the Google Products page speaks about a 'main product', but there is no individual set that is the main product. The entirety of the 8 is what makes it a complete set.

I tried turning on 'is_bundle' with 'yes', but eCRATER responds with an error that it's not supported. There is insufficient room in the UPC field to list all 8 UPC values.

Can someone tell me a legitimate set of attributes to 'keep Google happy' ?