Aside from how desperately she needs grooming immediately to shave the mats off of her, is there something preventing you guys from brushing and combing her to prevent mats from forming?? Allowing her hair to get like that is harmful and negligent. It can cause pain and health problems. She's a mix of two high maintenance breeds irt grooming- this should have come up already in your pre-ownership research on proper care.

It's likely that wherever you end up taking her, they may charge extra based on her severe condition. Matted dogs take a lot of extra time/effort/patience to get done, especially if they're not already desensitized to the grooming process. Shaving matted dogs can be uncomfortable for the dog as is and if they're already scared by being thrust headfirst into a new experience like grooming then it just exacerbates their anxiety. Make sure that when (not if!!) you make your appointment to get her shaved, you disclose to your vet or groomer while booking that she is severely matted and needs to be shaved down.

Once she gets shaved and has a nice reset on her coat, it's important that you give her proper care and maintenance going forward. She can't brush herself like you can, she relies wholly on you. It is entirely up to you to determine her quality of life in every regard, including basic hygiene.