Pella review: billing errors and customer service

We ordered replacement windows from Pella, Jacksonville, Florida back in September 2018. Our project manager was Shana Vixama. They installed the replacement windows on December 10, 2018 incorrectly, there was missing hardware and it was not installed as per the job order. We paid extra to have the replacement windows installed with nail fins with stucco repair. They had to remove the installed window and reinstall it on January 2019 together with the missing hardware. After that, I got a call from Shana to settle the remaining balance. A few minutes later she calls me to say she charged the wrong amount. I was then sent an invoice for $3912.00 as the final balance. I settled this balance on 01/28/2019. Fast forward 6months later, she calls saying we have an outstanding balance of $941 that needs to be paid immediately. She explained that this was due to a billing error made by the finance department. I understand I should have checked my contract before paying the bill to double check everything was correct but made the mistake that your employees would do so. We've already sold our house and have suffered loses during the sale. We are essentially being asked to make a payment for something we don't own anymore. We asked if they could help us with this in some way because we have not planned for this expense. We were then contacted by your company's collections coordinator Malanie LaDuke and she suggested we just charged this to our credit card which will give us 30 days. We proceeded to explain to her our financial situation and the fact that we could've planned for this during the sale of our house if they contacted us a lot sooner or did not make a series of billing mistakes. She proceeded to say it's none of her business and that mistakes like this do happen on their end and that other customers just pay the bill on similar situations. We feel that Pella and their employees should have some sense of business accountability and not hold their customers accountable for every mistake that they made. We understand that you're a big business and we're just the little guys who need your service but would appreciate if we are treated with some respect as consumers.