code: @NotNull
      public String getSku(@NotNull String appstoreName, @NotNull String storeSku) {
          checkSkuMappingParams(appstoreName, storeSku);
  
          Map<String, String> skuMap = storeSku2skuMappings.get(appstoreName);
          if (skuMap != null && skuMap.containsKey(storeSku)) {
              final String s = skuMap.get(storeSku);
              Logger.d("getSku() restore sku from storeSku: ", storeSku, " -> ", s);
              return s;
          }
          return storeSku;
      }
docs: Returns a base internal SKU by a store-specific SKU.
  
  @throws java.lang.IllegalArgumentException If the store name or a store SKU is empty or null.
  @see #mapSku(String, String, String)