It must be a standard letter. Reason:
1. The price of regular mail is very cheap; it costs only 60 RMB per letter when using discounted postage.
2. Unregistered and registered letters are now treated equally internally, both with barcodes, but registered letters are very expensive.
3. The shipping costs are too high, and there are also strict packaging requirements, which may require inspection.
Sending packages to a retail store is definitely inconvenient, so you can only send them in batches. If the location is very far away, you could also hire a cheaper courier service to deliver the package to the retail store, which would only require paying for the delivery once.