So the problem is your html code:
Your image size displayed is: 280px (width) × 70px (height)
The height of 70 px is given by your barcode parameter inserted (/h70/). (Default value is 100 px.)
The scale value given is the default one (/s1/).
It is because you enforce the image width to be 280 pix.
So it generates an homothetic distorsion, by stretching it, perhaps not convenient for scanner reading, anyway, giving this blur aspect.
Anyway, with the distortion, the height is enforced to 128px, bypassing the h70 you defined.
I tried it without the width attribute, and the image seems far clearer.
The problem is that using scale x2 generates a pretty larger image, probably not convenient for your email delivery:
286 (w) x140 (h)
To my mind, the better would be to suppress this width attribute.

Regards
JS