This isn't possible on BrickOwl. There are only two variables - parts and shipping - any additional fees need to be added to the shipping costs. There are a few options. You could set a minimum order value to avoid the issue entirely (though admittedly this might lose you some low value orders), you could enable only the Request a Quote shipping method and manually calculate the fees you wish to charge for each order (not recommended) or you could duplicate all your shipping methods to account for the extra fees on small orders. For example:
Europe (low value order) Maximum order value: €4.99. Shipping cost €3.50.
Europe (normal value order) Minimum order value: €5. Shipping cost €3.
Additionally, if you're a business (including sole trader) in the EU, there are now limitations regarding if and when you can charge extra when a customer selects a particular payment method.
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Europe (low value order)
Maximum order value: €4.99. Shipping cost €3.50.
Europe (normal value order)
Minimum order value: €5. Shipping cost €3.
Additionally, if you're a business (including sole trader) in the EU, there are now limitations regarding if and when you can charge extra when a customer selects a particular payment method.