I'm assuming Assurant is an AMC (appraisal management company - I don't recognize the name) But either way, they probably have a panel they use, the size of the panel varies from company to company. As the others have stated before, the appraisers may not want to take what they are offering to pay them. I almost guarantee the $515 you are paying is NOT going directly to the appraiser (if you do find out what they actually pay the appraiser, we/at least I would be interested to know, but don't know if they'll disclose that to you)
Another reason, besides fee, may be your property. Are you somewhat unique? (i.e. acreage in a mostly tract type area, larger home, smaller home, waterfront) This could also be a reason
All the best and hopefully someone reaches out to you soon