Songkran Atchariyasap, 39, filed a complaint with the DSI.
He said that Bang Buathong post office, where he placed an order for an amulet on Dec 19 and paid 999 baht, had refused to give him a refund.
Mr Songkran sought the refund after hearing news reports that the Royal Household Bureau had announced it not given the amulet producers the royal flowers they claimed were used as materials for making the taslimans.
The bureau also said it had not granted any permission to include an engraved image of the royal crown behind the amulets.
Mr Songkran also took legal action against amulet maker Sitthikorn Boonchim and his company Diamond Hill Co.
The source said the DSI investigators would also question representatives of state agencies mentioned as having roles in the production and commercial distribution of the amulets.
The source said the agencies to be questioned included Thailand Post, Krung Thai Bank, Thai Military Bank, Siam City Bank, and the private-sale distributors.
Representatives from the Royal Household Bureau, the Department of Religious Affairs, and the Office of National Buddhism would also be asked to give details about the amulets, the source said.
The DSI would also ask newspapers, billboard owners and other media outlets who placed the controversial Phra Somdej Nue Hua advertisements with them.