diff --git a/internal/invoice/invoice.go b/internal/invoice/invoice.go
new file mode 100644
index 0000000..f5cbf83
--- /dev/null
+++ b/internal/invoice/invoice.go
@@ -0,0 +1,33 @@
+package invoice
+
+import "fmt"
+
+// LineItem is a single billed item on an invoice.
+type LineItem struct {
+	Description string
+	AmountCents int64
+}
+
+// Invoice represents a generated invoice for a payment transaction.
+type Invoice struct {
+	Number string
+	Items  []LineItem
+}
+
+// Generate builds an Invoice for the given transaction ID from a set of
+// line items, auto-numbering it using the transaction ID.
+func Generate(transactionID string, items []LineItem) Invoice {
+	return Invoice{
+		Number: fmt.Sprintf("INV-%s", transactionID),
+		Items:  items,
+	}
+}
+
+// Total returns the sum of all line item amounts, in cents.
+func (i Invoice) Total() int64 {
+	var total int64
+	for _, item := range i.Items {
+		total += item.AmountCents
+	}
+	return total
+}