It sounds like your Register may have sent an automatic receipt. Automatic receipts are sent if a customer has previously entered their email to receive receipts from a merchant using Square, they're automatically subscribed to receive receipts at this email for all subsequent purchases from all Square merchants. Because this happens automatically, you wont be prompted to send a receipt.
And yes! You can always resend a receipt to your customer. To resend a receipt from your Dashboard:
# Sign in and click Sales > Transactions
# Use the date selector tool to locate a payment.
# Click on the payment> Send Receipt.
To resend a receipt from the app:
# Tap the three horizontal lines in the navigation bar> Activity.
# Locate and tap the payment> Issue Receipt.
