I'm looking for information on how to setup pay-in-slips on invoices. Basic idea behind it is to give customers the option to pay at the bank, 7-Eleven or ATM (using a barcode on the invoice).

Employees of bank branches tell me to call Bangkok, customer services then tell me to ask at their branch...and the circle is compete.

Counter Services Co.Ltd. has unfortunately no English website.

Anyone who has experience setting this up and willing to point me in the right direction?

You would be subscribing to a service called "bill payment". You pay a monthly fee as well as transaction fees, and get a specification for a bar code etc that you can print on your invoices. The exact prices will vary depending on your expected volume & you can choose whether to pay all the fees yourself or pass some of them on to your customers (i.e. the people who pay you). Your bills can be paid via atm, internet banking, call center and at branch counters nationwide. At Bangkok Bank you can call, 2230 1306 - talk to K Sudthida (thai) or K Sakkosol (Deow) who speaks good english.