Just to be clear it is the case that the free recruitment bonus comes in the form of cash credit added to the relevant accounts and not at free memberships.
So that bonus cash credit can then be used to purchase memberships, add-ons or pay for out of bundle services such as international calls/texts, MMS messages etc.
And, as I mentioned, if you have membership auto renewal switched on then that will always buy your next membership using the registered card and not any cash credit on your account.
If you want to use your bonus cash credit to buy a membership then you need to do this manually.
In that case the membership auto renewal will skip that month rather than buying a second membership.
Hope this helps.