hello,
i am going to create a site for a conference, and allow user to make payment online. what module should i use? i search from the modules in xoops and discover that the payment is only for credit cards or paypal, but is there any module that i allow the users Direct Debit to bank account ? and later produce a receipt to the user.
please advice, thank you.

You will be able to use XPayment and write a Gateway Plugin for your provider of direct debit etc. Nothing on XOOPS is in itself a title which directly offers you payment, however there are many thousands of providers out there of payment gateways which you would be able to customise a gateway plugin for.

And yes as it is GPL technically you are ment to email it to me when your done so I can include it in the distribution. This feature you talk about for the reciept of the direct debit, is called an IPN, it is when the Gateway provider dials your site with the invoice number and the payment result to notify you that the payment was recieve or rejected.

This with XPayment will fire a Plugin which you can customise for the module you are charging from with XPayment and it to mark the sale through and paid.