Thonky says 50,000 bells "in the shop", so I'm assuming they mean in-store items. In addition, it says that "only items that can be sold out count towards your total", which excludes stationary, wrapping paper, etc. and fortune cookies are counted as 500 bells spent. You can always re-order from your kiosk (excluding non-orderables, of course), so I'm assuming kiosk items don't count.
Also, Animal Crossing Wiki says you also need to have had the gardening centre and Super T&T opened for 21 days to get T.I.Y. in addition to the 50k spent.
