Majority of the Shipping labels are getting generated automatically by the Shipping Vendor, and there is no way to adjust the printed label.
As a workaround, an invoice template can be used to add additional details on top of the label, such as SKU and item quantity.
1. Go to Settings > Template Designer > Add Invoice Template
2. Change the size of your invoice to 4x6 inches.
3. Drag Shipping Label to your invoice and position it in the centre.
4. Add Order Items table to your invoice. Edit Order Items table (double click on it) by removing all the columns and headers except SKU and any others you wish to keep and position it where you want the values to be printed.
5. Print specific invoice from Open Orders by selecting your newly created invoice template.