YOU DO NOT NEED A WET INK SIGNED LETTER OF INTENT FROM BENEFICIARY TO BE MAILED, a photocopy of the signed document is ABSOLUTELY okay.
2 forms of proof of this - the official I129f instructions states this:
PROOF 1 of 2 - pg 2 of https://www.uscis.gov/sites/default/files/document/forms/i-129finstr.pdf
Each petition must be properly signed and filed. For all signatures on this petition, USCIS will not accept a stamped or typewritten name in place of a signature.
note: a signature is not a stamp, and it is not type written. Typewritten definition is to be a font from computer or typewriter.
PROOF 2 of 2 - from USCIS policy manual https://www.uscis.gov/policy-manual/volume-1-part-b-chapter-2 see valid signatures, it says it in plain English
A signature is valid even if the original signature on the document is photocopied, scanned, faxed, or similarly reproduced. Regardless of how it is transmitted to USCIS, the copy must be of an original document containing an original handwritten signature, unless otherwise specified. The regulations do not require that the person signing submit an “original” or “wet ink” signature on a petition, application, or other request to USCIS.
surfnjam got a reaction from Sut in K1 Fiance Visa Step-by-Step Guide on Filing an I-129F for a Foreign Fiance