Having a Parcel Collect or Parcel Locker address as "Primary" will also cause the payment button to be greyed out. 

The work around for Parcel Collect is to set a different address as "Primary", then edit the saved Parcel Collect address to remove the space between the two blocks of 5 digits.

After the edit, it can then be set as "Primary" again, and the payment button will become active.