Does your boss give you a thank you card for each day you work? If fifteen percent of the book's income for life isn't enough, I want a new agent.
Now, if a blizzard had everyone snowed under, and if a subsequent power failure meant nothing could be sent electronically, and if my agent walked the manuscript seven miles through a blinding snowstorm to get it there on time, I'd do better than a bottle of whiskey.
But an agent is getting PAID to sell your manuscript, and it's no different than a clerk in a story geting paid to sell a candy bar. Well, except that the clerk does keep getting paid over and over for as long as the book stays in print.
If yoir book sells, you will talk to your agent on the phone. B epolite and says thank you. But more that this is just not necessary.
If anything, the agent should be sendning the writer a gift for writing a book that makes teh agent money. It's just weird that the writer does the meaningful work, but the agent is supposed to get the gift.