Hi!
1) If we talk about user-defined delivery methods, then you can set up a weight condition in shipping rates like this:
http://awesomescreenshot.com/0de423ew74
If you mean real-time shipping rates, then it is not supported yet.
2) Correlation between payment and delivery methods is not added yet as well.
Please report these ideas to our ideas.x-cart.com tracker, so we would consider it during future development. If you need these features right now, feel free to order a customization from us. Get a free quote for such task via
sales@x-cart.com