I don't want to show my ads in other apps right now, but accumulate the credits and use them later. Can I do this?

Unfortunately you can't accumulate credits on AdDuplex. This is done to preserve the balance on the network. If we would've allowed that, quite a few publishers would accumulate credits and there would be no ads to show on some days, but then everyone would decide to use their accumulated credits before/after some holidays and there would be no ad space to show everyone's ads.

So, the credits you get for displaying ads have to be used right away (this is done automatically).