Thank Them, and Ask Them to Thank You _ must be sincere Money is often a touchy subject, so politeness about it is a good idea. Your clients are paying you money that they've earned with blood, sweat and tears, so let them know you appreciate it. You should also invite them to contact you if they have any questions and, more importantly, make it clear that you appreciate their present (and future) business. Some people also welcome testimonials; for example, by adding, "Let us know how we did. Write a testimonial and sent to... " If you're building your website's testimonials page or want to complete the feedback loop, this is a great way to get clients to give feedback on your work. If they have suggestions for making the process smoother, it's also a great opportunity for you to improve.
If you are permitted to submit your invoice via email then it is useful to convert the invoice into PDF format so that it cannot be altered. There is a free PDF creator which you can use at PDFCreator. This creates a PDF file from any application which can then be emailed to the client. Better off, if you can use a software system, it will automatically generate invoice in PFD format, you can also email it straight away from within the a software program.