A very good question! I will try to explain it...
"I held the phone away from my ear, puzzling about where missionary luggage-christmas-gift zeal stemmed from."
Missionary Zeal = A "Missionary" is a religious person who tells other people about their religion. "Zeal" is 'a lot of enthusiasm,' or 'a lot of excitment,' a lot of happiness.
Since missionaries are very excited about their religion, they can work very hard to tell other people how good their religion is, etc.
You can describe anyone as a missionary if they are very excited about telling a lot of other people about... anything.
Tom: "David, you have to listen to this music by Gwen Stefani! She is fantastic! Her music is the best in the world! I am telling EVERYONE about how good she is!"
David: "Um, when did you become a missionary for Gwen Stefani?"
(You are as excited about her as a religious missionary is about God.)
Bridget does not know why her mother is so excited about luggage for Christmas. So she is almost making a joke about why her mother has become a missionary for.. luggage as a Christmas present? Why is her mother as excited about luggage as a real missionary is about God?
Does that help?
PS - Gwen Stefani's music IS the best in the world.
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