you mentioned the implementation details (-> default value is passed, boolean value is checked in delegate method), but in interface view this knowledge should be encapsulated. so
you should not pull up the implementation-details to the interface.
i tend to try to keep interfaces tidy and keep "redundant helper"-methods out from the interface.
if i can say from the interface point of view (design by contract) that method A() can be easily simulated by B(), just by passing a different parameter i would parametrize my method A(...), so in your case i would parametrize and only place one method in interface:
public void processInvoice(InvoiceData invData, boolean otherParameter)
but just like that, otherParameter has not a very good naming (steve already mentioned importance of namings), so clients would not instantly see, what it means and could get confused. in the end you could define interface (assumed boolean flag is for validation):
interface Invoice{
...
public void process(InvoiceData invData, boolean skipValidation)
...
}