Hi Marco:

I roughly understand your needs,  and I think the module "delivery carrier" might be referenced:

there are 2 sets of pricing mechanism, one is fixed price, and the other is based on rules which can be customized.  Once you select the "base on rules" for a certain carrier,  at the frontend the delivery cost will automatically match your rules and be calculated, without showing the other scale prices.

I haven't studied the source code of such module yet, so just propose a concept for your reference：

• the rules actually maintained in an indiviual model,  with fields used for evaluation factor(such as weight,dimensions, order amount etc.);
• then in the delivery carrier model,  a one2many field mapping to the rules model as above;
• and then the type of pricing mechanism,  it could be a selection;
• lastly i think there might need some logical method to determine how the final price will be.