Transaction amount isn't exposed during callback

[coatesap]

Currently it seems impossible to actually retrieve the transaction amount from Worldpay's callback. This is important so that a merchant can check that the amount processed hasn't been tampered with (which does seem to be relatively easy with Worldpay).

If this is genuinely outstanding, and I haven't missed something, I'll be happy to provide a pull request.

[delatbabel]

I'm not aware of any means to do that so if you can provide a PR that would be useful, thanks.

[coatesap]

We've actually worked around the problem of tampering in our system, but this issue should probably be left open, as it would still be a sensible change for someone to implement.