I don't think Xfinity has a Showtime package that doesn't also include HBO (the way, say, Xfinity has Showtime in its 300-channel level but HBO is only in its 450-channel one). I think it's an extra $20 per month for this.
@Missaray wrote: Does anyone know the cost to subscribe to Showtime?
See https://www.xfinity.com/support/rate-card/. In my area (south-central PA) Showtime is $12/month.
Actually, I do vaguely remember being given the option to include Showtime separately when I first switched - I forgot about it because I eventually decided to go for the Premier level when they included installation for free. However, since I already have it, I can't access that information now.
Here's a way that might work:
Sign in to your online account, then go down to where there is a button marked "Manage Premium Channels"; it should bring up a page that shows all of the available premium channels (Showtime, HBO, Cinemax, etc.) and what they cost per month if you want it separately.
Keep in mind that if you want more than one channel, it may be cheaper to upgrade to Digital Premier.