Non-static method cannot be referenced from a static context

(SalesService.java)

@Service
@RequiredArgsConstructor
public class SalesService {
  private final SalesRepository salesRepository;

  public void saveSales(String title,
                        Integer price,
                        Integer count,
                        Authentication auth) {
    Sales sales = new Sales();
    sales.setCount(count);
    sales.setPrice(price);
    sales.setItemName(title);
    CustomUser user = (CustomUser) auth.getPrincipal();
    var member = new Member();
    member.setId(user.id);
    sales.setId(user.id);
    sales.setMember(member);
    salesRepository.save(sales);
  }
}

(SalesController.java)
  @PostMapping("/order")
  String postOrder(@RequestParam String title,
                   @RequestParam Integer price,
                   @RequestParam Integer count,
                   Authentication auth) {
    SalesService.saveSales(title, price, count, auth);
    return "redirect:/list/page/1";
  }

SalesController.java에서 saveSales에 빨간 밑줄이 그어지고 Non-static method 'saveSales(java. lang. String, java. lang. Integer, java. lang. Integer, org. springframework. security. core. Authentication)' cannot be referenced from a static context 라는 에러가 뜹니다. 코드를 어떻게 고쳐야 할까요?