A cash-in code is just a request to add up your marks and give you an overall grade. You only submit a cash-in code when you have completed a qualification - so if you are entered for only 2 units they you can't. There also isn't a cash-in for A2, only for AS and A level. As all the AS units fall within the A level which ones they use for it are irrelevant. In fact, if you're taking A level FM then there's little point cashing-in the AS at all, but if you do they will calculate it using FP1 and the highest 2 other possible units.
Cash-in doesn't exist in England any more and I'm not that up to date with international qualifications, but it used to be the case that you also needed to (re-)cash-in your Maths AS and A levels when you cashed in FM to allow movement between qualifications to maximise your grades - e.g. you might be able to get better grades by swapping around some of M1, M2, D1, S1 and S2 between Maths and FM (Maths can contain 2 from D1, M1 and S1 or a pair of 1 and 2, e.g. M1 & M2.)