Is it permissible to accept a sum of money in exchange for preparing and customizing simple, readily available online computer systems that save companies money and time, given that this work does not require significant effort or time, and the financial compensation might be substantial compared to the simplicity of the service?

The work you do for your expertise and knowledge is permissible, for expertise has a price and value, and it increases the remuneration of its owner. There is no limit to profit, unless it involves an exaggeration beyond the market price of similar items, or exploitation of those who are unaware of market prices. This has been stated by Ibn Uthaymin and the Permanent Committee, citing Allah's saying: ﴿And Allah has permitted trade﴾ and ﴿unless it be a trade by mutual consent among you﴾. Nevertheless, it is preferable for a Muslim to be lenient in his selling and buying, due to the Prophet's (peace be upon him) saying: "May Allah have mercy upon a man who is lenient when he sells, and when he buys, and when he demands his rights."