TL;DR: Whichever is fine, but using くる is safer.
You seem to know the basic rule, but just to make sure, this type of verb after
te-form of another verb + て/で is called a subsidiary verb, and they are generally written in hiragana.
But this rule is not very strict, and every so often you will see people use kanji for subsidiary verbs, especially in stiff and stilted writings.
However, verbs after the te-forms are not always subsidiary verbs. きた in 暖かくなってきた is definitely a subsidiary verb, while 来た in 彼は急いで来た ("He came in a hurry") is a normal verb, so it is usually written in kanji. 見た in 箱を開けて見た ("I opened the box and looked (inside it)") is used as a normal verb, while みた in 箱を開けてみた ("I tried opening the box") is used as a subsidiary verb.
So the problem is this: Is きた in 荷物を送ってきた a subsidiary verb or a normal verb? How about きた in 荷物が送られてきた?
Well, I believe these きた can safely be considered as a subsidiary verb. デジタル大辞泉 clearly defines these usages under the 補助動詞 section:
So the safer choice is to use hiragana for both of your examples.
But I also feel using kanji is not so unnatural in these cases, because something is physically moving. People often use the kanji 来る even in casual situations when something is physically moving to you. I personally am inclined to use hiragana for those cases, but perhaps this is a gray-area matter and it's up to the writer's taste.