You sell websites built on Wordpress, Drupal, Prestashop...
You offer hosting on your server with Linux, Apache, MySQL...
You need Free Software.
Free Software needs you.
Free and open source software enables tens of thousands of people to market products and services in a short time and at reduced costs. It allows many, probably including you, to build their own professional activity only on their own skills.
But software, as you know, does not grow on trees. And also the open source software, though often distributed for free, has a cost.
The financial support for open source projects, and their related initiatives, is not just a matter of morality but a way to ensure that your business can continue to grow in the future. Without depending on expensive and restrictive licenses, binding development environments, or the will of others.
On you invoice add 1% to the grand total using, as describing item, "Opensource contribute - wwww.linux.it/onepercent". In this way your customers will be made aware that software you used to complete the job, although open source, need support in order to stay sustainable and continue to enjoy it.
If you want you can eventually not include the invoiced fee, or define a fixed or variable amount to your liking; the important thing is to collect, in fair and responsible way, funds for the development, update, maintenance and improvement of your own productive tools.
Each month, each year, each invoice: you choose when and how distribute the collected money. Below, some quick reference to donate to the most popular projects.
Do not forget the many plugins and modules for CMSs and frameworks you use, often having their own donation page!
Do I have to explicitely add a "opensource contribute" amount on my invoices?
Absolutely no: that is a suggestion but there are no rules to adhere to this initiative, nor in the mode neither in collected amounts. You can just add a note on the footer of your document, like "1% of the profit will be devolved to opensource projects". Clearly stating the contribute on your invoices is useful to inform your customers about the "cost" of free software, but it is intended as a pure educative and symbolic message.
Can I have tax exemption on the donated money?
Most of the above mentioned web pages have informations about tax exemption, which are specific for each foundation and organization. Please note that cross-country donations are usually non-exemptable.