How can I turn on ‘save on postage’ only rather than % off?

Wondering if anyone can help! Anyone know how I can have save on postage only rather than a percentage off bundles?

My items are very cheap anyway and I don’t want to have a percentage off a few but I don’t want buyers to pay multiple times postage.

Can anyone help? The options on the enable bundles toggles is only certain amounts off, and there is no zero option.

Thank you :)