Hi there,
The ME3 store has had a long history of issues and some of these issues has resulted in lost credits in the past. Part of the issue is the convoluted nature of the Store, in that an active, functional connection to both the EA/Origin and ME3 MP servers. Any hiccup with the EA server connection and BOOM you're booted back to the main screen with no DLC, at best, and crashed out at worst. That being said, since the recent changes to store, you should now get any purchases presented next time you log in. I find that after any ME3 crash it's best to restart both Origin and ME3: it seems to reduce ongoing stability issues.
If you don't get your purchase next time you log in and you made the purchase with Bioware points, then there is a transaction history. When I recently contacted EA support about lost in-game credits transactions, I was told there is no transaction history for in-game credits(I would suggest that the Bioware points purchases are tracket with an accounting style double entry ledger, but the credits are simply tracked in a counter and there is no ledger, as it were).
Truth is, while ME3 is an awesome game, it has a long history of bugs, and many bugs go un-fixed to this day (for exsample SMG ultralight materials and the Krogan Battlemaster's final rage evolution, which have remained un-fixed, and worse, are still available in-game despite being bugged). The very worst situation is when two issues interact: for example the, original, poorly written store code interacting with the unreliable EA/Origin servers. Mass Effect is impacted by the most common issue of all: just because someone can write code to achieve an outcome doesn't mean they can write the best code. For the store system they needed to hire some hard core web-devs and DBAs, instead they seem to have hired people who could do the job, just not as well as the community may have preferred.
Here are some BSN forum topics for you:
http://social.bioware.com/forum/1/topic/343/index/13831958
http://social.bioware.com/forum/1/topic/343/index/13681628 - The Bioware store is, at the end of the day, a slot machine, afterall
I hope that helps!