Hello there, @Beancey.
At this time, QuickBooks Online (QBO) only allows you to add bundle items for sales transactions. Thus, adding it to a purchase order (PO) isn't available.
As a workaround, you can consider adding each product or services under your bundle items to PO manually.
I know how worthwhile it is for you and your business to add bundle items to PO. However, the products and services in QBO are working as designed.
For now, you can visit our blog site so you'll be able to get the latest news about QuickBooks and what our Product Care Team is working on.
Feel free to leave a reply in the comment section if you've got more QuickBooks concerns. Take care and stay safe always.