BPM using Method Directive Maintenance

I want to create a BPM that gets the Delivery Days field from the Customer Ship to Tab and then subtract that amount to the Need By Date of the Sales order line and update the Ship by date.

NOTES: We are on a Multi-Tenant SaaS environment so no custom code allowed, we need to use Method directives. I already have the bpm working if I hardcode the difference, I used BO Sales Order, method MasterUpdate.

I just need to know how to access Customer table and fetch that value.

Thanks!

I also tried adding a variable type Customer Tableset

But I got this error

There is at least one compilation error.
CustomizationAdapter.cs(613,30): error CS0433: The type ‘ETCAddrValidationTableset’ exists in both ‘Erp.Contracts.BO.Customer, Version=10.2.200.0, Culture=neutral, PublicKeyToken=5d3fa3c7105d7992’ and ‘Erp.Contracts.BO.SalesOrder, Version=10.2.200.0, Culture=neutral, PublicKeyToken=5d3fa3c7105d7992’
CustomizationAdapter.cs(9,142): error CS0738: ‘SalesOrderSvcCustomization’ does not implement interface member ‘SalesOrderSvcContract.ETCValidateAddress(int, int, int, out bool, out bool, out string, out bool)’. ‘SalesOrderSvcCustomization.ETCValidateAddress(int, int, int, out bool, out bool, out string, out bool)’ cannot implement ‘SalesOrderSvcContract.ETCValidateAddress(int, int, int, out bool, out bool, out string, out bool)’ because it does not have the matching return type of ‘ETCAddrValidationTableset’.