We bought an acrylic tub 20 in in height and bought OS&B waste/overflow clicker that fits 20 in tubs.. but it doesn't fit. Is it normal to cut the pipe to size? It doesn't make sense to me especially when the kit has a flexible copper pipe on overflow for perfect fitting.

20"deep tub is deeper than regular 16" tub. Regular tub takes 12" overflow. Therefore, you should have purchased overflow that fits your tub. They sell them in 3 sizes. Even better, you should purchase overflow that is supplied with ABS or PVC fittings + trim - but you supply the pipe which you cut to length according to the depth of your tub. Looks like they sold you wrong size overflow. Back to you. Milo