Can I share iCloud storage without also agreeing to pay for iTunes purchases? We have four phones on the same plan, including those our children use (who are adults).  We're running low on iCloud storage.  Apple messages said we could get 50MB each for $.99 a month or 200MB that could be shared.  But now it appears that the sharing would also entail agreeing to pay for iTunes and a number of other things.  How can we share just iCloud storage, as Apple seemed to tell us we could do?
No. Family sharing in spring 2018 and back to when it started implies one payment card for all purchases. 

The only exception of course is store credit or gift card balance can be applied to any or all parts of a family so that most purchases pull from that prepaid balance. So if you manage it or push out “allowances”, most of the time, only recurring payments and IAP hit the shared card.