How big a box? ECS used to sell carpet sets that were shipped laid flat never folded. Claim was you could never get all the folds and creases out of carpets when they'd been shipped rolled and folded up.
I never saw one of their carpets in its packaging but it must have been a big box!
I was told the trick with carpets is to order the carpet, then as soon as it arrives, unpackage it and lay it flat somewhere. Do not store in its box for any length of time!
The carpets are made to order and therefore when you receive an order, it almost certainly hasn't been in the box for more than a few days. Therefore if you lay it out right away, any distortions from being folded up haven't had time to become permanent.
The last carpet I installed was over 10 years ago, an ACC mass backed carpet for one of my Challengers. I laid it out as soon as it arrived, and it fit like a glove with no flaws. It took a while for my hands to recover from all the cutting and trimming of that thick product. It was worth the pain though!