They can scan by selection too, prices are a little higher (as it takes more work).
Scans from selected negs will cost $1.00 each. Mininum of 10 images. Thereafter, each image will cost $0.70. This is for 1818 x 1228 scans. For the higher res (3360x2240) scans, it will cost quite a bit - $2.50 per scan for 10, $1.50 thereafter.
You can combine different film strips into a single roll of ~36 and scan for $12 (1818x1228) or $28 (3360x2240), which might save you some money.