What are the pitfalls when storing data retrieved from an api
Advice:


Normalize data stored.  
Consider carefully and figure out your write model.  For orders, can you overwrite?  do you need to mark superceded and append? 


The above are generic data entry pieces of advice, but regarding automated application to application interfaces, you have the issue of erroneous data entry.  To deal with this it is probably better to accept orders etc. but have the invoicing done by humans, who can cross check with the real world.  Also this keeps people in control of your books.  So I would add to that:


Separate automated entry from homan review and approval.