diff --git a/src/Rest/Category.php b/src/Rest/Category.php
index 3600f12..2415199 100644
--- a/src/Rest/Category.php
+++ b/src/Rest/Category.php
@@ -4,17 +4,17 @@
 
 final class Category
 {
-    public static string $GIFT = '1';
+    public const GIFT = '1';
 
-    public static string $COMMERCIAL_SAMPLE = '2';
+    public const COMMERCIAL_SAMPLE = '2';
 
-    public static string $COMMERCIAL = '3';
+    public const COMMERCIAL = '3';
 
-    public static string $DOCUMENT = '4';
+    public const DOCUMENT = '4';
 
-    public static string $OTHER = '5';
+    public const OTHER = '5';
 
-    public static string $MERCHANDISE_RETURN = '6';
+    public const MERCHANDISE_RETURN = '6';
 
     private string $value;
 
diff --git a/src/Rest/OutputFormat.php b/src/Rest/OutputFormat.php
index a2f8bfd..59a5cde 100644
--- a/src/Rest/OutputFormat.php
+++ b/src/Rest/OutputFormat.php
@@ -4,14 +4,14 @@
 
 final class OutputFormat
 {
-    public static string $ZPL_10X15_203DPI = 'ZPL_10x15_203dpi';
-    public static string $ZPL_10X15_300DPI = 'ZPL_10x15_300dpi';
-    public static string $DPL_10X15_203DPI = 'DPL_10x15_203dpi';
-    public static string $DPL_10X15_300DPI = 'DPL_10x15_300dpi';
-    public static string $PDF_10X15_300DPI = 'PDF_10x15_300dpi';
-    public static string $PDF_A4_300DPI = 'PDF_A4_300dpi';
-    public static string $RETURNTYPE_SENDPDFBYMAIL = 'SendPDFByMail';
-    public static string $RETURNTYPE_SENDPDFLINKBYMAIL = 'SendPDFLinkByMail';
+    public const ZPL_10X15_203DPI = 'ZPL_10x15_203dpi';
+    public const ZPL_10X15_300DPI = 'ZPL_10x15_300dpi';
+    public const DPL_10X15_203DPI = 'DPL_10x15_203dpi';
+    public const DPL_10X15_300DPI = 'DPL_10x15_300dpi';
+    public const PDF_10X15_300DPI = 'PDF_10x15_300dpi';
+    public const PDF_A4_300DPI = 'PDF_A4_300dpi';
+    public const RETURNTYPE_SENDPDFBYMAIL = 'SendPDFByMail';
+    public const RETURNTYPE_SENDPDFLINKBYMAIL = 'SendPDFLinkByMail';
 
     private int $x = 0;
 
diff --git a/src/Rest/Service.php b/src/Rest/Service.php
index 523857b..b29dd66 100644
--- a/src/Rest/Service.php
+++ b/src/Rest/Service.php
@@ -4,33 +4,33 @@
 
 final class Service
 {
-    public static string $A2P = 'A2P';
+    public const A2P = 'A2P';
 
-    public static string $BPR = 'BPR';
+    public const BPR = 'BPR';
 
-    public static string $ACP = 'ACP';
+    public const ACP = 'ACP';
 
-    public static string $CDI = 'CDI';
+    public const CDI = 'CDI';
 
-    public static string $CMT = 'CMT';
+    public const CMT = 'CMT';
 
-    public static string $BDP = 'BDP';
+    public const BDP = 'BDP';
 
-    public static string $PCS = 'PCS';
+    public const PCS = 'PCS';
 
-    public static string $DOM = 'DOM';
+    public const DOM = 'DOM';
 
-    public static string $DOS = 'DOS';
+    public const DOS = 'DOS';
 
-    public static string $BOS = 'BOS';
+    public const BOS = 'BOS';
 
-    public static string $BOM = 'BOM';
+    public const BOM = 'BOM';
 
-    public static string $RECOMMENDATIONLEVEL_R1 = 'R1';
+    public const RECOMMENDATIONLEVEL_R1 = 'R1';
 
-    public static string $RECOMMENDATIONLEVEL_R2 = 'R2';
+    public const RECOMMENDATIONLEVEL_R2 = 'R2';
 
-    public static string $RECOMMENDATIONLEVEL_R3 = 'R3';
+    public const RECOMMENDATIONLEVEL_R3 = 'R3';
 
     private string $productCode;