How should I be adding free shipping as an option on subsequent items?

I have a number of listings for items which can be posted together. In the past I've been able to have one of the shipping options as "$0" "add to current order", the idea being that the customer chooses the appropriate postage rate for one item and the rest can be added without any extra fees.

This has worked for about 20 years, but recently it has been flagged as a problem as there isn't an option in the postage pop-down for zero payment.

I'm now unable to relist a number of my listings (not all of them, it's a bit random). Does anyone have a work around for this?

Cheers!