I got your point , let me give this information to you.
A) You have to raise PR for payment and once gets Approval in SAP System with put the PGr is P29 (P29 means purchasing dept in Thailand), No need to send email with supporting document because system automatically routing to purchasing dept. Then K.Sumalee or purchasing dept. will send soft file of purchase order to you by e-mail.
B) Once you gets the PO, you have to attach PO and original Invoice together and send to ALISA, CHUASENA (Mut) to doing payment process.
C) Once ALISA, CHUASENA (Mut) receiving your invoice already she will scanning done and input in the system (Payable), System will automatically send send e-mail to Approval again for confirm and release payment.
D) At last you have to follow up with ALISA, CHUASENA (Mut) on the status of payment.
I think it not convenience with you about this process because,
1. Almost contract and quotation is in Thai languages and before you will buy something ,it must be send your requirement to purchasing dept. to finding and negotiate price first
2. Account dept or payable dept need original invoice and original billing note together with purchase order for pay out only.