Venue upcharge

In July 2020 we picked a venue and set up a contract involving a payment plan. Part of the contract was that the venue would be paid in full by 30 days before the wedding day. We paid off our venue in full making our final payment In December 2020 (7 months early). On Thursday 6/24 the venue sent an email stating that sales taxes increase on 7/1/21 for the parish. Therefore all events after 7/1/21 will be charged an additional 0.05% towards sales tax. Attached to the email was an invoice stating we now owe $972.74 that needs to be paid in full by Monday 6/28. The venue is closed to the public Friday, Saturday, and Sundays for weddings. Is this a legitimate and/or legal thing to be billed for an additional amount due to increase in sales tax after the services were paid in full? Especially considering the increase in tax is only 9 days before the event and even if we did not already pay in full months early our last payment would have been due an entire month prior to increase in sales tax.
I plan to send an email tomorrow to have a paper trail. Because We both work and are unable to take off Monday I plan to call and speak to someone for clarification on this matter. Just looking for advice considering this came out of left field with a super short notice and we are both shocked and not sure how to go about handling this matter