It's possible that you're searching for the wrong thing. If you...
- are only selling one service (with customisable variations);
- require no product catalog;
- need a 'service customisation form' upon checkout;
- don't want users to have to create an account;
- need PayPal/payment processing;
- want a control panel to check who's paid and log their details;
...then I suggest that you don't want a shopping cart at all, or even an 'eCommerce platform for service providers'. I suggest that you evaluate online form-building services that support payment upon submission instead.
Wufoo sounds like it would suit your needs. It's a hosted online form builder, and their $29.95/month 'Bona Fide' account supports payment upon form submission. Here's how you'd set it up to accept payment from users buying a service:
- Your website includes a description of the service and a 'buy now' link.
- The buy now link takes visitors to a custom Wufoo form (hosted on their site or embedded on yours) to collect details. No registration is required.
- The visitor fills the form and gets redirected to a payment page automatically upon form submission.
- Regardless of whether or not the visitor completes the payment, you get an optional email notification or SMS to alert you of the form submission.
- You, or customer service people, can log in to the Wufoo control panel, see a list of all submissions, check that payment has been submitted, and add comments against the user's submission (that they can't see) to track progress of each 'job', generate reports and more, using a simple backend.
Wufoo has an example of a product payment form here. Their example uses physical goods, but it works just as well for services. Try filling the form and submitting it to see the payment page, which is also customisable. (You don't have to actually buy anything to try it out.)
I've set this up for two clients, both of whom run consulting businesses, and they've been very happy with the workflow it gives them.
Formstack is another web-based form builder with payment support that you might like to consider.