Hi James, try to make a field related first in the sale.order.line model

Note that this code is in v8 odoo

#declare the selection first
PRODUCT_TYPE = [('consu', 'Consumable'),('service','Service'),('product', 'Product')]
#declare the field
field_related = fields.Selection(PRODUCT_TYPE, related='product_id.product_tmpl_id.type', string='Fields used for attrs', readonly=True)

#on the view.xml
button name="print_report" string="Print" type="object" icon="gtk-print" attrs="{'invisible':[('field_related', '!=', 'product')]}"/>.

Regards,