diff --git a/amazon/validator.go b/amazon/validator.go
index 1d0b3fe..0861c1b 100644
--- a/amazon/validator.go
+++ b/amazon/validator.go
@@ -7,7 +7,6 @@ import (
 	"fmt"
 	"net/http"
 	"os"
-	"time"
 )
 
 const (
@@ -25,13 +24,6 @@ func getSandboxURL() string {
 	return url
 }
 
-// Config is a configuration to initialize client
-type Config struct {
-	IsProduction bool
-	Secret       string
-	TimeOut      time.Duration
-}
-
 // The IAPResponse type has the response properties
 type IAPResponse struct {
 	ReceiptID       string `json:"receiptId"`
@@ -57,34 +49,31 @@ type IAPClient interface {
 type Client struct {
 	URL     string
 	Secret  string
-	TimeOut time.Duration
+	httpCli *http.Client
 }
 
 // New creates a client object
-func New(secret string) IAPClient {
-	client := Client{
+func New(secret string) *Client {
+	client := &Client{
 		URL:     getSandboxURL(),
 		Secret:  secret,
-		TimeOut: time.Second * 5,
+		httpCli: http.DefaultClient,
 	}
 	if os.Getenv("IAP_ENVIRONMENT") == "production" {
 		client.URL = ProductionURL
 	}
+
 	return client
 }
 
-// NewWithConfig creates a client with configuration
-func NewWithConfig(config Config) Client {
-	if config.TimeOut == 0 {
-		config.TimeOut = time.Second * 5
-	}
-
-	client := Client{
+// NewWithClient creates a client with a custom client.
+func NewWithClient(secret string, cli *http.Client) *Client {
+	client := &Client{
 		URL:     getSandboxURL(),
-		Secret:  config.Secret,
-		TimeOut: config.TimeOut,
+		Secret:  secret,
+		httpCli: cli,
 	}
-	if config.IsProduction {
+	if os.Getenv("IAP_ENVIRONMENT") == "production" {
 		client.URL = ProductionURL
 	}
 
@@ -92,19 +81,16 @@ func NewWithConfig(config Config) Client {
 }
 
 // Verify sends receipts and gets validation result
-func (c Client) Verify(ctx context.Context, userID string, receiptID string) (IAPResponse, error) {
+func (c *Client) Verify(ctx context.Context, userID string, receiptID string) (IAPResponse, error) {
 	result := IAPResponse{}
 	url := fmt.Sprintf("%v/version/1.0/verifyReceiptId/developer/%v/user/%v/receiptId/%v", c.URL, c.Secret, userID, receiptID)
-	client := http.Client{
-		Timeout: c.TimeOut,
-	}
 	req, err := http.NewRequest("GET", url, nil)
 	if err != nil {
 		return result, err
 	}
 	req = req.WithContext(ctx)
 
-	resp, err := client.Do(req)
+	resp, err := c.httpCli.Do(req)
 	if err != nil {
 		return result, fmt.Errorf("%v", err)
 	}
diff --git a/amazon/validator_test.go b/amazon/validator_test.go
index 681bc0d..77e272d 100644
--- a/amazon/validator_test.go
+++ b/amazon/validator_test.go
@@ -59,10 +59,10 @@ func TestHandle400Error(t *testing.T) {
 }
 
 func TestNew(t *testing.T) {
-	expected := Client{
+	expected := &Client{
 		URL:     SandboxURL,
-		TimeOut: time.Second * 5,
 		Secret:  "developerSecret",
+		httpCli: http.DefaultClient,
 	}
 
 	actual := New("developerSecret")
@@ -72,10 +72,10 @@ func TestNew(t *testing.T) {
 }
 
 func TestNewWithEnvironment(t *testing.T) {
-	expected := Client{
+	expected := &Client{
 		URL:     ProductionURL,
-		TimeOut: time.Second * 5,
 		Secret:  "developerSecret",
+		httpCli: http.DefaultClient,
 	}
 
 	os.Setenv("IAP_ENVIRONMENT", "production")
@@ -87,40 +87,20 @@ func TestNewWithEnvironment(t *testing.T) {
 	}
 }
 
-func TestNewWithConfig(t *testing.T) {
-	t.Parallel()
-	config := Config{
-		IsProduction: true,
-		Secret:       "developerSecret",
-		TimeOut:      time.Second * 2,
+func TestNewWithClient(t *testing.T) {
+	expected := &Client{
+		URL:    ProductionURL,
+		Secret: "developerSecret",
+		httpCli: &http.Client{
+			Timeout: time.Second * 2,
+		},
 	}
+	os.Setenv("IAP_ENVIRONMENT", "production")
 
-	expected := Client{
-		URL:     ProductionURL,
-		TimeOut: time.Second * 2,
-		Secret:  "developerSecret",
+	cli := &http.Client{
+		Timeout: time.Second * 2,
 	}
-
-	actual := NewWithConfig(config)
-	if !reflect.DeepEqual(actual, expected) {
-		t.Errorf("got %v\nwant %v", actual, expected)
-	}
-}
-
-func TestNewWithConfigTimeout(t *testing.T) {
-	t.Parallel()
-	config := Config{
-		IsProduction: true,
-		Secret:       "developerSecret",
-	}
-
-	expected := Client{
-		URL:     ProductionURL,
-		TimeOut: time.Second * 5,
-		Secret:  "developerSecret",
-	}
-
-	actual := NewWithConfig(config)
+	actual := NewWithClient("developerSecret", cli)
 	if !reflect.DeepEqual(actual, expected) {
 		t.Errorf("got %v\nwant %v", actual, expected)
 	}
@@ -175,6 +155,6 @@ func testTools(code int, body string) (*httptest.Server, *Client) {
 		fmt.Fprintln(w, body)
 	}))
 
-	client := &Client{URL: server.URL, TimeOut: time.Second * 2, Secret: "developerSecret"}
+	client := &Client{URL: server.URL, Secret: "developerSecret", httpCli: &http.Client{Timeout: 2 * time.Second}}
 	return server, client
 }