Additional validation parameters

[altenstedt]

We might consider adding more parameters that either require validation, or that should not be passed at all. Right now, it is perhaps a bit too obvious that the cost parameter is wrong. If we had additional parameters for how many products that is added to the basket, the discussion might also become a little more involved: there are other, more non-obvious parameters that need validation also.

However, we might complicate the API too much by adding additional parameters. Let's try it out and see what it looks like.