Need Long-term PHP developer for e-commerce product

I'm seeking to work with a PHP developer in the UK who has experience working with custom software such as e-commerce products. I've been working as the designer as well as company founder for 4 years full time. I have around 32 customers, 3 of which use the e-commerce platform.

I effort with a number of small businesses around my local town as I'm their website provider. They all have a continuing licence to the software which helps them sell their products. This software is a modest custom e-commerce PHP software which has a responsive front end interface. I want a developer to support and upgrade it at appropriate times throughout the year.

The developer can either
a) be paid for each upgrade, separately

b) be paid for each licence I sell

c) be paid for each licence fee received

d) open to suggestions

The software has been developed over time also it has a minimal footprint. The back-end interface has been designed from the ground up specifically to work well with tablets (iPads).

Desired Skills - MySQL Administration, HTML, PHP, Database, MySQL Programming