Seeking Information about Ishop services

I have a mailbox at Ishop in San Antonio. If someone in the U.S. sends mail or a package to the Texas address for Ishop, will it automatically be sent (brought?) to the Ishop in San Antonio? If so, is there an extra charge? And if so, is there any way to estimate how long it will take to get from Texas to San Antonio in Mexico?

I have time sensitive mail coming to me from the U.S. If Ishop service isn't the best and fastest way to get it, does anyone have an alternative suggestion?

Any quick help or suggestion will be greatly appreciated.

The issue is, Mail versus Packages.

Flat mail with no checks or cc's inside usually make it in 2 weeks whether it's Ishop or HandyMail or Sol y Luna.  My last few cc's made it thru fine, but others were not allowed, no rhyme or reason to it.  

For time sensitive, use a carrier like FedEx, DHL or UPS.  I have had my banks send cc's this way with 2 day service, never an issue.

Packages....there are many many Aduana limitations on what can be sent to MX legally.  Then there are items that are allowed in, but only after taxes/fees are paid to Aduana.  

Since you have a box there, why not ask Ishop your specific q's about what your mail or package contains?