Without a Certifying Acceptance Agent (CAA), you must mail your passport to the IRS (notarized copies are not allowed). This method can take several months
, which means that you will not have access to your passport during this time. In addition, the IRS uses untrackable shipping, which has led to a large number of missing and lost passports.
Requests for additional information are sent via mail, so you may receive iti close to or past the response date if you reside outside the U.S. Furthermore, you will have to read and figure out the appropriate information/documents to send in response to the notice and call the IRS if you have questions.
Filing an ITIN application without a CAA can take a minimum of 2-3 months for processing
Filing through Uncle Sam Inc, a CAA, can cut the processing time in half