[Duugu]

Hm. Wowprogramming says

numItems, totalItems = GetInboxNumItems() Returns: •numItems - Number of mails in the player's inbox (number) •totalItems - Total number of items both in the inbox and on the server. (number)

I'm not sure what the difference is as I never did something with the mail api.
Totally guessing: the first one is the number of mails the player had opened at least once, and the second one is the number of opened and unopened mails?

You could try:

local function Accountant_DetectAhMail()
    local numItems, totalItems = GetInboxNumItems()
    for x = 1, totalItems do    
        local invoiceType = GetInboxInvoiceInfo(x)
        print(x, invoiceType)
        if string.find(string.lower(invoiceType), "seller") then
            return true
        end
    end
end

Does this change anything?