How can I sort packing slip by SKU in Liquid?

[Bo-Dee-Oh]

I've used the board to determine how to sort my Packing Slip by SKU since that is how my products are stored.  However, the logic built into my SKU is causing me issues.  I'd like to truncate the first three characters of my SKU and sort on the remainder, as below.

AA9A1234

I've tried sort: {{'sku' | slice: 3, 5}} instead of sort 'sku' in my

{% assign lineitems = line_items_in_shipment | sort: 'sku' %} statement.

I'll admit I'm a rank beginner with Liquid 

TIA