To set a specific shipping cost, add =SS= to the item's part number so it bypasses your normal shipping rates. Next, code the per-item shipping charge into the Individual Item Shipping Cost field (field #6 of your item string), which appears right after the Quantity field.
For example, to charge $50 shipping, change:
a-1358^goldwidget^Gold Widget^99.00^1
to:
a-1358^=SS=goldwidget^Gold Widget^99.00^1^50.00
The $50 charge will be added for each quantity that customers purchase. In the example above, it will cost $150 to ship three gold widgets.