Please help me figure out what I have done wrong!

When I take my answer [18,909] and add 400 I get the right answer [19,309], but why do you add the shipping cost to the end? Was I supposed to add it earlier in the transaction process?

A company uses a perpetual inventory system to record the following events involving a recent purchase of inventory:

Aug. 1: Purchased merchandise for $20,000, terms 1/10, n/30

Aug. 3: Paid freight costs of $400 on merchandise purchased

Aug. 6: Returned $900 of merchandise to the supplier

Aug. 9: Paid the amount due to the supplier.

As a result if these events, the company's inventory is:

Da on perpetual Aug. 1 = 1/10, 1/30 purchase inventory 20.000 A/P 20,000 Aug. 6 Return a A/P 90o 3 Aug 3 (A Inventory 900 Shipping inventory 400 USERUOD Cash 400 00 ET 19, 100%,01 = 191 Aug 9: Du A/P 19,100 Cash 18, 909 inventory 191