Photoshop Elements 2021 has a number of tools (I think there are currently 11 🙂 ) that can be used to select subjects. You can then resize the subject, place the subjects on a different background, and then save the final image in a suitable format for use on the Web. There's even a Guided Edit that will help you step-by-step to replace the background.
Regarding cost, Photoshop Elements 2021 is a one-off purchase of around $99 and to upgrade to the latest version every year it costs around $79; Photoshop CC is available through subscription via the Creative Cloud Photographer's Plan for $9.99 per month, but all updates are included as part of the subscription (and you get Lightroom and a bunch of other very useful services from Adobe).
But more importantly, there are a number of other much more important factors you've got to consider than cost alone:
- How many product shots will initially need to be edited?
- How difficult will the selections be? e.g. are the products plants, cuddly toys, bicycles, plates, or cardboard boxes? Were the products photographed in front of a plain background (maybe a green screen)? Every product or product type will usually require a different approach to selecting the subject - some selection tools will produce better results than other tools, and some subjects will take longer depending on how much the selection needs to be refined.
- Were the product shots photographed under the same conditions? e.g. to get a professional-looking set of uniform final images you will also need to consider color correction and lighting.
- How often will their website be updated? i.e. how many new product shots will need to be created over time?
- Does your customer (or one of their employees) have any experience of using Photoshop or Photoshop Elements?
Are they glass or clear plastic, i.e. transparent or semi-opaque? They'll have to take into account the background being partially visible through the bottle, depending on the bottle's contents.
Solid bottles shouldn't be too complicated - depending on the shape of the bottle and the complexity of the background.
Why not download a 30-day trial copy of Adobe Photoshop Elements 2021 and give it a go yourself with some of the customer's product shots? Then you'll be able to go back to them with a rough idea of how much it would cost per product shot. The customer can then calculate how much money they're going to need to set aside every year for you to produce the product shots, versus doing it themselves for free but with the added overhead of the cost of the software, the training that will be involved to get them up to scratch with the software, and the time that they will have to dedicate to this task every year (which means less time for them to run their business). You might need to remind them that if the website (and related social media accounts) are their only sales channels this isn't something they can afford to do "on the cheap", so to speak 🙂