refactor code

src/main/java/com/peanut/modules/book/service/ExpressFeeService.java

@@ -0,0 +1,20 @@
+package com.peanut.modules.book.service;
+
+import com.baomidou.mybatisplus.extension.service.IService;
+import com.peanut.modules.book.entity.ExpressFee;
+import org.springframework.stereotype.Service;
+
+import java.math.BigDecimal;
+
+@Service
+public interface ExpressFeeService extends IService<ExpressFee> {
+    /**
+     * 计算快递费
+     *
+     * @param expressCompanyCode 快递公司代码
+     * @param weight             重量
+     * @param regionCode         地区代码
+     * @return BigDecimal 费用
+     */
+    BigDecimal calculateExpressFee(String expressCompanyCode, int weight, String regionCode);
+}