You'll want to buy them, but you'll need to get more involved with Liza first. If you haven't already, you'll need to finish progress in these activities as far as the game lets you:
- Get her phone at night and learn her unlock pattern.
- Get the video on her phone to show why she's ashamed.
- Blackmail her with the video. Speak with her in her room (Tue [late morning], Thr [noon], Fri [afternoon]) and make her clean your room while wearing a maid outfit. When she finishes whatever work you want, be sure to tip her when offered (increases her Affection, up to a certain point). It used to increase Domination as well; I haven't retested from earlier progress in the new version.
- When she sneaks into your room at night at first, just watch her. After that, tackle her to the floor. This will be possible multiple times. (IIRC, this may also increase Domination, but I'm not 100% on that.)
- As #3 above, but take pictures of her (for your "school project") in sexy, somewhat humiliating poses (increases her Domination, up to a certain point).
- When offered during #3, have her clean your lights. After she falls, you'll have an opportunity to show concern about her back. You want to get to where she'll let you look at her back and massage it (to make it feel better; no increase in Domination or Affection). Do this every time you're able. Eventually, the in-game guide tells you to try to massage her back in the living room.
- I'm not sure, but I think you might need to be able to massage her feet in the living room (need over 17 Affection) before continuing. If that's true, make sure Affection is high enough. (You can repeat #3 to increase Affection if needed, rather than doing #5.) When you're able, choose to massage her back.
Note: You want the option that reads "forced", not the option that reads "BAD". The "BAD" option will not take you further in her progress. If you only see the "BAD" option, you're not far enough along in progress with her yet.
- Like earlier times when she sneaked into your room, she'll try again. You'll get the idea to make a trap for her.
At this point, you'll be able to buy the handcuffs.