As I understand it, once you've calculated your credit total in the Gain Credits part of the Requisition phase, that credit total is then fixed until you spend it in the following part of the phase. That's how I did it, anyway - I had two copies of...Shrewd Collector, I think it's called, a Boost which says to gain an artifact thingy for every fifteen credits you have. So I worked out my credit total, then after that but before buying stuff, used the Boosts to get a pile of the artifacts (thus spending the Shrewd Collector cards - had I done it before then I wouldn't have had their credit values to add to my total).
There is nothing that says you can only use one Boost at a time so I presume I did this right, spending multiple Boosts at once. Incidentally, as you can only spend a Boost in between Events...if you spend a Boost after the last Event of a Phase, does it count as being part of that Phase or the next one? Say I had a Boost which could only be used during the Requisition phase, could I use it after the last Event of that phase or would we be considered to have moved onto the Installaion phase by then?