fix(shipping): round weights before posting to shippo

Shippo's /v1/orders endpoint rejects >2 decimal places on parcel
weight and >4 on line_items[].weight. Converting ounces to kg
(oz * 0.0283495) routinely produces 6-decimal values, so the first
real label purchase 400'd with "Ensure that there are no more than
N decimal places".

Round at the payload boundary in createShippoOrder rather than in
the caller — keeps the precision of the source (Coffee.Ounces) and
only loses it when the wire format demands it.

pkg/shippo/shippo.go

@@ -183,7 +183,7 @@ func (c *Client) createShippoOrder(ctx context.Context, to Address, email string
 		lines = append(lines, map[string]any{
 			"quantity":    item.Quantity,
 			"title":       item.Title,
-			"weight":      item.WeightKg,
+			"weight":      roundN(item.WeightKg, 4),
 			"weight_unit": "kg",
 		})
 	}
@@ -195,7 +195,7 @@ func (c *Client) createShippoOrder(ctx context.Context, to Address, email string
 		"to_address":  toPayload,
 		"line_items":  lines,
 		"placed_at":   time.Now().Format(time.RFC3339),
-		"weight":      totalKg,
+		"weight":      roundN(totalKg, 2),
 		"weight_unit": "kg",
 	}
 	var resp shippoOrderResponse
@@ -304,6 +304,19 @@ func addrPayload(a Address) map[string]any {
 	return m
 }
 
+// roundN rounds to n decimal places. Shippo's /v1/orders rejects >2 decimals
+// on parcel weight and >4 on line-item weight.
+func roundN(v float64, n int) float64 {
+	pow := 1.0
+	for i := 0; i < n; i++ {
+		pow *= 10
+	}
+	if v >= 0 {
+		return float64(int64(v*pow+0.5)) / pow
+	}
+	return float64(int64(v*pow-0.5)) / pow
+}
+
 func parseAmountCents(s string) int {
 	f, err := strconv.ParseFloat(s, 64)
 	if err != nil {