Should i replace my already booked mua vendor?

So, here's the scoop..

My MUA received my retainer deposit November 2020. I never received a full invoice or even a means of understanding the payment breakdown. I reached out very early February 2021 to try to better understand really what I need to do in order to not be behind on any owed payments. The platform they contact me through says I haven't paid my retainer even though I have confirmed they have received payment.
I've already attempted to reach out regarding adjustments that need to be made for my services. Nothing. Long story short, 3 emails withing 30 days and I never heard back. I took it upon myself to gather more quotes until they responded to me. Doing so, I found a MUA who offers the SAME services and quality at approximately 500 bucks cheaper.
Now, the original vendor has reached back to me. They claim my email was missed but, that's still a problem for me because they sign didn't really address my concerns besides just telling me what I originally requested. But it took weeks to get back to me?I've conflicted. Should I consider this unacceptable and breakup and move on to a cheaper MUA or continue with the original at a higher rate?