荷受人からX運送業者への指示は特に何もないと思います。
私は通関手続きについて詳しくはありませんが、あなたの言う輸送中の部品は無税になるというのは理解できません。
荷受人はこれまで何度も部品を注文していますが、毎回輸入税を支払っているはずです。
（後払いができるため、いつも荷受人はY運送業者を指定します）
私たちもあなたから送られる予備部品の輸入税を毎回支払っています。
今回はX運送業者へ輸入税の前払いが必要のため、荷受人から立替払いを頼まれました。
I think there wasn't any particular instructions from the package receiver to the sender.
I don't know customs procedures that well but I can't understand why you say that the parts that you say are being exported don't have tax.
(It can be paid after delivery therefore the receiver will always identify the Y sender vendor.
We have been paying the importation tax for the spare parts that are sent from you.
This time, it's necessary to pay the importation tax to X sender vendor in advance. Therefore we were asked by the receiver to pay in their behalf.
