SagePay error affecting US customers

Hi Guys,
I hope you are all well it seems a long time ago since I had to post anything here but I am up against the clock on this one, I have a live site which I used ECart5 it was built at the beginning of last year, We have started getting problems relating to US sales, I believe it is to do with the Sage Pay Protx_VSPForm.php

Sage Pay error: Status: MALFORMED 3147 : The BillingState field is required,

I have examined the error and found that several other shopping cart vendors have encountered the same issues and have released fixes,
I have purchased ECart6 but I am currently working full time as a front-end developer for a digital agency based in London and as the cart is quite heavily customised I do not really have the time to run through the whole process again.

I have inspected the Protx_VSPForm.php form and believe the best solution would be to add the BillingState field and associate it with the value from the confirmation page for Billing address State/ County (state_province) and pass this across to Sage Pay, (at the moment I believe we are storing that value in the dB and retrieving it when we get the call back from Sage Pay.)

As the site is live I need to apply a fix quickly and efficiently I have attached the files for your attention please could you take a look and advise me of the best fix for this.

Thanks in advance

Steve,