I transmitted my form electronically but still need to send form 8805 (baffling the Turbo Tax system didn't simply give me a way to upload the PDF). Where do I mail it?

Form 8805, Foreign Partner’s Information Statement of Section 1446 Withholding Tax, will be mailed to the following address

Internal Revenue Service
P.O. Box 409101
Ogden, UT 84409

See the link below directly from the IRS website

https://www.irs.gov/filing/where-to-file-forms-beginning-with-the-number-8

I am already sending the form to that address when I file my business taxes. 
Could I include an extra form with a note? 
Do I have to send it in a separate letter? 

Send it in a separate letter, to avoid it getting lost in processing.