Sending mail is not so difficult. You may use services like UPS or DHL, as well the mexican post offers express services.
If You want to ship goods it is abit more conplicated but up to a value of about 1000 Dollar, it is not a big problem. The receiver must pay some taxes.
And yes, there is a company which uses a US Shipping adress and forwards to Mexico. It is called MERKALINK.
I myself bought some items in USA and shipped them through them. But I had to pay a gould amount of fees.
Hope this helps.