JSP752 does state that you can add gratuities to the bill but only at a rate of 10% (reference is something like 03.0111 - how sad is that!).
The civil servant's regulations states that you can only use PIE for certain things as well but is administered with much more common sense. I believe their system actually automatically adds in PIE for every day they are away.
Where do I stand if an advance that JPA says has been paid to me, but never arrived in my bank account, is taken back off my wages next month? Is that theft?