Round amounts to the cent the same way on macOS and Linux

README.md

@@ -91,6 +91,11 @@ try await client.refund(transactionID: txnID, amount: 49.99,
 try await client.void(transactionID: txnID)
 ```
 
+Amounts are `Decimal` and are sent with exactly two decimal places (`40` becomes `"40.00"`). An
+amount with more than two decimal places is rounded to the cent, half-to-even, the same way on
+macOS and Linux: `49.995` is sent as `50.00` and `49.985` as `49.98`. If your prices can have
+fractions of a cent, round them yourself first with the rule you want.
+
 ## Result
 
 ```swift

Sources/PerfectAuthNet/AuthNetClient.swift

@@ -193,13 +193,20 @@ public struct AuthNetClient: Sendable {
     }
 
     /// Formats a `Decimal` as a fixed two-decimal string (`"49.99"`), locale-independent.
+    /// Amounts with more than two decimal places are rounded half-to-even (`49.995` → `"50.00"`,
+    /// `49.985` → `"49.98"`), the same on every platform.
     static func formatAmount(_ amount: Decimal) -> String {
-        let formatter = NumberFormatter()
-        formatter.locale = Locale(identifier: "en_US_POSIX")
-        formatter.numberStyle = .decimal
-        formatter.usesGroupingSeparator = false
-        formatter.minimumFractionDigits = 2
-        formatter.maximumFractionDigits = 2
-        return formatter.string(from: NSDecimalNumber(decimal: amount)) ?? "0.00"
+        var value = amount
+        var rounded = Decimal()
+        NSDecimalRound(&rounded, &value, 2, .bankers)
+        // `Decimal.description` always uses "." and never groups or uses exponents.
+        var text = rounded.description
+        let fraction = text.firstIndex(of: ".").map { text.distance(from: $0, to: text.endIndex) - 1 }
+        switch fraction {
+        case nil: text += ".00"
+        case 1?:  text += "0"
+        default:  break
+        }
+        return text
     }
 }

Tests/PerfectAuthNetTests/PerfectAuthNetTests.swift

@@ -115,6 +115,28 @@ struct PerfectAuthNetTests {
         #expect(AuthNetClient.formatAmount(Decimal(string: "0")!) == "0.00")
     }
 
+    @Test(arguments: [
+        ("49.995", "50.00"),   // half-to-even rounds up to the even cent
+        ("49.985", "49.98"),   // and down when the cent is already even
+        ("49.9951", "50.00"),
+        ("49.994", "49.99"),
+        ("0.004", "0.00"),
+        ("0.005", "0.00"),
+        ("0.015", "0.02"),
+        ("1234567.891", "1234567.89"),
+        ("100000000000000000000", "100000000000000000000.00"),
+        ("-12.345", "-12.34"),
+    ])
+    func amountFormattingRoundsToCents(input: String, expected: String) {
+        #expect(AuthNetClient.formatAmount(Decimal(string: input)!) == expected)
+    }
+
+    @Test func amountFormattingIgnoresCurrentLocale() {
+        // Decimal(string:locale:) with a comma locale still yields the same value; the output must use ".".
+        let amount = Decimal(string: "1234,5", locale: Locale(identifier: "de_DE"))!
+        #expect(AuthNetClient.formatAmount(amount) == "1234.50")
+    }
+
     @Test func stripBOMRemovesLeadingMarker() {
         let withBOM = Data([0xEF, 0xBB, 0xBF]) + Data("{}".utf8)
         #expect(AuthNetClient.stripBOM(withBOM) == Data("{}".utf8))