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Payment: These types of vouchers is a proof of payment by cheque or cash. Receipt: These vouchers help in the recording of the receipts. If the payment is received from a customer, then a voucher will be prepared as a proof to the transaction. Purchase: This voucher is made as a proof to the purchase. This helps in determining whether the entity has actually carried out the purchase.
Invoice is what one would bill for payment with. Voucher is an internal document or documents the Government uses as the evidence to pay a bill. You might try looking in your agencies finance regualtions, policies, handbooks etc. as I suspect 'voucher will be defined there somewhere.