How can I allocate freight costs to the inventory items purchased?

enter the bill for the receipt of the items as you normally would

when the freight bill comes in, enter it and use an expense account called clearing expense, create it if you need to first

then edit the first bill, for each item increase the total cost by a portion of the freight amount, then change to the account details portion and select the clearing expense account, enter the amount of that bill as a negative number

insure the original amount of the bill has not changed and save, click through any warnings about payments being applied if you get them

Thanks for getting back, Gosia. 

Yes, you'll have to enter the freight bill as a bill. After, follow the steps above given by Rustler, with regards to the account you'll use. 

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