Asalaw, thank you - my use of the word "invoice" was incorrect. An "invoice" is what Wah would have sent to Desilu for payment. ("Remittance Advice" is even written by the document number.) (1)
BTW - the Desilu Productions Inc., Remittance Advice for Wah's "reworking [of the] phasers" was number: 43977 and was paid 25Sep1966.
Wah's Vendor number was: 0407.
Wah submitted invoice number: 7761 on 22Aug1966.
He received $520.00.
And apparently Wah charged Desilu $1,019.20 for "designing and making two practical and eight dummy communicators". (2)
(1) Star Trek Sketchbook: The Original Series, page 238
(2) Ibid 1, page 239