When a sales order is pushed from Salesforce to Netsuite the Bill To always default to custom. How do i make it so its always billing?

Do you have address records marked as Default Billing Address on the customer? If so, they should pick up the Bill To automatically. If that's not the case, there could be a mapping in your integration that's setting it to custom. Either way, if the standard functionality isn't working that selects the Default billing address automatically, you should be able to set that in your integration.

Thank you. how can i check if its defaulting to custom?

If you have a Default Billing Address set on the customer record, the system should never default Custom as the option, so I'm guessing it's something with your integration setup. You'd have to have whoever has access (if not you) go in to the mapping of the integration and see if it's hard-coding custom as the option.

Thank you so much. You were right its the integration