Too easy!!
That's the one week of the year when you can judge pretty much what they're going to cost you. Look back over the last few years, they don't vary that much, perhaps go up 20p on the big ones.
They haven't picked Val Day because they want to do it on the cheap, so it's probably not a big issue, bit like going out to a nice restaurant when you could buy all the stuff at Tesco and do it yourself. It's their choice, not yours.

Given that you're a professional, you take the same care as you will with all the other orders you get for that day, so it's no different to someone phoning you now and ordering a hand tied for their wife for Valentines. Would you reject it? Course not. If it was something intricate and time consuming, that would be different.

Just price it accordingly. If they don't like the price, then you can reject it.

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