I figured this out a while ago, but since this is still open I will update with an answer:

In website_sale/controllers/main there is a method called pricelist with route /shop/pricelist

I inherited this controller in a custom module and replaced the part

request.website.sale_get_order(code=promo, context=context)

with

request.website.sale_get_order(code=promo.upper, context=context)</code>

And then I inherited product.pricelist and its 'create' and 'write' method

 class product_pricelist(models.Model):
    _inherit = "product.pricelist"
    @api.model
    def create(self, values={}):
        '''
        Force the pricelist-codes (couponcodes) to be uppercase
        '''
        if values.get('code'):
            values['code'] = values.get('code').upper()
        res = super(product_pricelist, self).create(values)
        return res
    @api.multi
    def write(self, values={}):
        '''
        Force the pricelist-codes (couponcodes) to be uppercase
        '''
        if values.get('code'):
            values['code'] = values.get('code').upper()
        res = super(product_pricelist, self).write(values)
        return res

This makes sure that whatever you enter as a promocode in the backoffice when configuring, it will always be uppercase