Trouble using Max() function
You cannot get a single MAX value for end_date while you're grouping on lines_0_description because each billing record will, presumably, have a different description producing a different distinct group and different max end_date values.

You can use a CTE to extract the most recent billing record per customer then join that to your main SELECT to produce the result set you want. Something like below should work (disclaimer: I've guessed at some column names, e.g. _id in the billing table, as I don't know the schema)

;WITH BillingRecords AS (
  SELECT MAX(_id) AS billing_id,
    customer_$id
  FROM billing
  WHERE (billing.lines_0_description like '%package1%' 
       or 
       billing.lines_0_description like '%package2%' 
       OR 
       billing.lines_0_description like'%package3%')
  GROUP BY customer_$id
)


  select customer.addresses_0_firstname AS AdminFName, 
       customer.addresses_0_lastname AS AdminLName, 
       customer.addresses_0_organisation AS CompanyName, 
       customer.addresses_0_address1 AS Address1, 
       customer.addresses_0_address2 AS Address2, 
       customer.addresses_0_city AS City, 
       customer.addresses_0_postcode AS Zip, 
       customer.addresses_0_country AS CountryID, 
       customer.addresses_0_email AS Email, 
       (CASE WHEN billing.lines_0_description like '%Package0%' 
             THEN 'False' 
             ELSE 'true' END) AS Reseller, 
       billing.end_date
from customer 
INNER JOIN BillingRecords br ON br.customer_$id = customer._id
INNER JOIN billing on billing._id = br.billing_id
where customer.name='example' 
  and (billing.lines_0_description like '%package1%' 
       or 
       billing.lines_0_description like '%package2%' 
       OR 
       billing.lines_0_description like'%package3%') 
group by customer.addresses_0_firstname, 
         customer.addresses_0_lastname, 
         customer.addresses_0_organisation, 
         customer.addresses_0_address1, 
         customer.addresses_0_address2, 
         customer.addresses_0_city, 
         customer.addresses_0_postcode, 
         customer.addresses_0_country, 
         customer.addresses_0_email