I'm currently looking at creating a new orders system using AWS, including SQS, Lambda, DynamoDB and RDS. Is there any benefit to having the basic order details stored in DynamoDB for the workflow process to pick up, and storing a more detailed version of the order with billing information etc in RDS for reporting/invoicing? Or should I just use RDS and skip DynamoDB?
This is very dependent on your application requirements. Do you require predictable low latencies when fetching order details?
If you need fast responses along with some of the easy integrations which DynamoDB provides, such as streams, event bridge, lambda etc... then it makes sense to use DynamoDB.
If you are not so concerned about speed, and integration then i'm sure you will suffice using RDS.
Remember with DynamoDB you get export to S3 and incremental export to S3 features built in, which can often handle reporting or monthly analytics. DynamoDB also has ZeroETL connectivity to Redshift.