Use custom variable to define transaction price

[aleau]

Hi, 

I want to define a rate plan for an API product which I need to set a transaction price according to a custom variable I got from the backend (with a Service Call). The number of call have no interrest here

Ex :

If the variable < 20000, the transaction cost will be 2,45

If the variable > 20001, the transaction cost will be 2,10

So I created a rate plan like following :

Rate plan type = Rate card

But I got an error "Metering type - STAIR_STEP is not supported for rating based on custom attribute and UNIT is not supported for rating based on aggregation of custom attribute." on saving.

Is there something I'm missing, or it's not the right way to configure my rate plan.

Thanks and best regards,
Alban