Can I rule out ER+?

No, you can't.  Whether you had a hysterectomy has no bearing on the percentage of receptors on your breast cells - which is what the measurement of estrogen positivity is.  It is not a measurement of the estrogen level in your body.  Even though you had a hyst/ooph your body still produces estrogen - by way of androgens produced in the adrenal glands and body fat.  This is why post-menopausal women take aromatase inhibitors - these drugs block the enzyme (aromatase) that converts these androgens into estrogen, and can fuel the estrogen receptors on your breast cells.

I am not sure since I have a prescription plan - I take generic Femara (letrozole), as I also had a total hyst/ooph many years ago, and I pay a $5 co-pay every 30 days for mine.  Do you have any prescription coverage?  If not you might check at pharmacies in big box stores like Costco, or at Walmart or Target - sometimes their drugs are less expensive.  If you are paying out of pocket for these prescriptions you may also want to price check both the three different aromatase inhibitor drugs (Femara - letrozole, Arimidex - anastrazole, and Aromasin - exemestane), and different makers of the generics for pricing.  You might also check Express Scripts mail order.