any original one invitation letter required for a Mexico client?

Yes, for business visa application in Mexico, original invitation letter is required.

Then, if the original one coming late, what will do. does the scanned one workable?

I am afraid not, visa may be denied if without original invitation letter. So, you'd better send it to our client asap. However, in some other countries, scanned copies, Fax are acceptable. Or you can ask your client to contact the embassy for detials.

Thanks so much dear Lassie. thank you, take care.