You need to have 100 dollars and then use your computer in your room to order a swimsuit online. The swimsuit will be delivered the next day and that will get you the achievement. Sadly that is the only thing the swimsuit does. It does not change the event at the pool with your sister and grandmother.
I avoided Kendra but i opend the Parcel and there was no money in it. Just pictures of MC and used panties.I know this scene was in the Limited Edition at the time so if you don't have that patch, maybe you can't get it? It should just be the Lexi scene, where you just take all the horniest options. It's definitely important to look at her breasts when turning around and saying they're almost too fake.
You buy the swimsuit from the PC. You need 100 buckaroonies for it, so you should either take Kendra's money and not give it to Sasha, or open the parcel that gets sent to you rather than giving it to Sasha (you can open it and then give it to her though).
With the amount of secrets and achievments the game kinda wants MC to share/be shared and be cucked