To Purchase this Sefer 
One who sends Kosher Tzitzis strings to a friend through a gentile messenger is required to place two signs on the strings, as explained in Yoreh Deah 11 [regarding the laws of Kosher food]. This however only applies if there is good reason to suspect that the gentile will benefit from exchanging the strings for another pair, however if he will not benefit at all from the switch, then we do not suspect that perhaps the gentile will make the switch in order to cause the Jew to stumble [and hence in such a case the strings may be sent without any signs]. Likewise, the above only applies when sending strings that are not yet interwoven, or when sending the interwoven Tzitzis string in its long state, prior to it being cut to four strings. If however one sent the four cut interwoven strings, there is no need to suspect that perhaps the gentile will switch the strings. [Practically today, one may send Tzitzis strings through the mail and it is not required to contain two signs.]
One may send Tzitzis strings through a gentile and it is not required to contain two signs.
 20/3; M”A 20/1; Beir Heiytiv 20/4; Aruch Hashulchan 20/4
 The reason: As we suspect the gentile may exchange the strings for another pair of strings that was not woven for the sake of Tzitzis. [ibid]
 20/4; M”A 20/1
The reason: As certainly the gentile does not have similar strings to these strings as such strings are only usually made for the sake of Tzitzis. [ibid]
 Piskeiy Teshuvos 20/3