Did they see examples of your own work before they booked/paid you? What I mean to say, is did they send you a picture of a cake from a magazine (with straight writing, crisp corners etc.) and agree to your price, and then they receive a nice cake, but not perfect like a cake from a magazine? If I knew what I was getting into (beginner cake decorator) and agreed to a price, then I suppose I would be ok with that cake. If I was expecting something perfect, then I would be really disappointed. That said - for a beginner, your cake is pretty good. Very ambitious though! Perhaps work on perfecting your tiered round cakes (achieving smoother fondant and straight lines) before taking another square cake order.