Some words with complex Kanji are sometimes written in a mixture of Hiragana and Kanji. For example 処方箋 (prescription) is often written as 処方せん in pharmacies. This is used by native speakers that have clearly received a Japanese education and can read Kanji, yet they still use Hiragana in some cases.

What are the reasons for the use of Hiragana here? Are Japanese people able to read Kanji like 箋? What contexts would they use Hiragana instead? Why is it acceptable to use Hiragana mixed with Kanji but not Katakana?

Is this kind of usage limited specifically to signs in stores that can be read at a distance or is it written in kana in other contexts as well?