Yes, our invoicing software Bookipi allows you to request signatures from your clients before you send them an invoice. You will need to ensure that the 'Get client signature' option is turned on when creating your invoice in order to enable this feature.
Your client will need to sign the invoice in order to be able to mark the invoice as paid when they receive it, so they will need to sign it then. As soon as the invoice has been signed by the client, you will receive a notification that the invoice has been signed, and you will be able to download a copy of the signed invoice for your records once it has been signed by the client.