You're a freelancer. A client of yours finally pays a bill — seven months late! — via email money transfer.
Should you send him a thank-you email? Or should you simply be silent?
(We acknowledge when someone's done something for us. We even acknowledge inanimate objects for their contributions: e.g. Moses didn't strike the land since it'd hid the Egyptian for him. But I don't know how far hakarat ha'tov goes. Does it extend to cases where someone's inconvenienced you, or when you may inadvertently be training him that wrong behavior is okay?)
Please cite sources if possible.
Dear readers: CYLOR.
P.S. If you do future work for this client, perhaps you should insist on full payment in advance.