Question about customs tax

[Victor]

I am still trying to understand the Canary ways, and I must admit—it is harder than I expected. 

One thing: I am buying from Amazon Prime - EU vendors etc. 
I am often asked for the paperwork, and my parcels stay for a month in Gran Canary. 
Sometimes, they ask for custom tax (again, for EU products coming from the EU), and I usually pay a bit more than 25% of their value. 

Is this legal? I understand there is a small VAT here (7%); it makes no sense why I need to pay so much tax. 

Why is this happening, are the residents here happy with this?

Where is this Money going? (25% tax for most products consumed on the island is a huge, huge amount)

[Johnrgby]

I buy quite a bit via Amazon, and other online retailers, and never pay anything extra, yes customs in Madrid and Gran Canaria are slow or very slow but customs tax no, I always incorporate my NIE number and telephone number into my address, which I find helps.

[Victor]

Hi John, all deliveries over 150 e are taxed. E
ven for a 130 e package, I had to pay over 30 e taxes. 

I hope someone can help us understand this.