Even it is guaranteed, the person who brings the package to you is not the one you talked to, so this is an amira deamira before shabbos and by paying extra reflects a great need
If there isn’t actually a great need, paying extra doesn’t make it muttar. The Minchas Yitzchak said that not as a heter going forward, but rather as a limud z’chus for those who were sending packages for Shabbos delivery.