This started as a few thoughts but turned into a humongous critique.
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In general.. there's a whole hell of a lot of solid colours in here for something that should be so dead-simple for clients to understand. Cutting them down might be an idea, becasue right now it's really hard to digest with all the colours. Try cutting it down to two.
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You should make the "Pay Invoice" button, as well as the due date, a very prominent call-to-action in two areas: beginning of the invoice (near the price), and the end of the invoice. Right now, you have it at the end of the invoice yeah, but it's pretty subtle and doesn't draw my eye's attention immediately. Will probably lead to longer wait times for the freelancer to get paid. Also, perhaps make the "due on X" copy a bit more serious.
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Your attachment layout is really confusing - I read the "4 Products & 13 Attachments" headline and immediately tried to figure out where the attachments were, and got even more confused that they weren't laid out for me to see gro