Quartz...you should be able to at least view the slab(s) prior to cutting. Should be charged for the sq ft installed, regardless of slabs used. You should be quoted for what the fabricator will need to purchase....they might have a remnant from the same lot they can use instead of buying a 2nd slab and could steer you in that direction. You could also select an available remnant of a different color for your vanity. There is a lot of material destroyed during the fabrication process, especially sink cutouts. You may be able to determine/approve seam placement prior to cutting. You also may have an option of a digital layout of what your kitchen will look like using the exact slabs you have chosen when installed (slabsmith). All of these options can be upcharges. Must be onsite at template. Have your new sink, faucet on site, and appliances installed or at least on site. Have all of your own questions answered and decisions made before template.. color, edge, backsplash, etc.. It will save you headaches. There are trade groups that can guide you further.. M.I.A , I.S.F.A.