In your E-junkie Seller Admin > Cart Sales Tax & VAT Settings > Advanced Tax Settings, you can enter tax rates according to zipcode. These should be specified as the total tax rate; more-specific locations/zipcodes would override (rather than add to) the rates for general locations, and you can even specify partial zipcodes to cover a whole range of zips.
E.g., if you configured Simple Tax Settings to Tax Buyers from State: North Carolina with a 7.75% rate and then added these Advanced Tax Settings:
282,8
2820,8.5
28201,8.75
...then buyers in NC would have 7.75% tax added to their cart total of items with Tax enabled, except for buyers in zipcodes 28200-28299 who would have 8% tax added, except for buyers in 28200-28209 who be taxed at 8.5%, except for buyers in 28201 who would be taxed at 8.75%.