Thank you for the post and welcome to Community Forum! We really appreciate your suggestion; we will refer this to our developer team. You can also try to send an online invoice link to make sure that they have received the invoice. The help article, Sending an online invoice link to a customer, has detailed information and instructions to assist with this.
Since @Isaiah_C and @Shoaib1 already provided you further information and option on how to make sure that your customer and/or client will receive their invoice, if they resolution provided by them answered your enquiry, please don't hesitate to click the "Accept as Solution". Let us know if you need further assistance.