One time charge

[Tool Pusher]

We considered the low order fee but we would probably need more than one tier. The shipping is by weight which of ocurse parts usually don't weigh much.

[gjh42]

If your shipping fees are calculated by weight, add a tare to small packages so the weight will never be less than your minimum cost to handle.
Configuration > Shipping/Packaging >
Package Tare Small to Medium - added percentage:weight 0:3

You can also use the low order fee at another breakpoint if you need more weighting at the lowest end. It doesn't all have to exactly equal the carrier charge, as long as it averages out for you and doesn't give any big unexpected spikes to the customer.